* We can't use system anymore in preview 3.  This introduces a lot of issues around envrionment variables, which can't be listed in swift :-(.  So we no longer inherit environment variables, we only set the ones we set.
    * We do pass on PWD and PATH, because otherwise that would be terrible
* Toolchain is now a part of platform config instead of something we pass around by hand.
* We now detect xcode 7 / 8 depending on whether we're using a toolchain installed to Xcode.app or Xcode-beta.app.  That's still not right, but fuck it.

This implements, more or less, the scaffolding described in #36.  This commit does not actually make tools behave differently in any configuration (so this is mostly a placebo) but after this, tools can adjust their behavior.

Notable additions/departures from the original proposal include:

* Support for "custom" configurations outside the built-in set
* The addition of `test` and `bench` as built-in configurations, since 2/2 developers use them
* Internal API has new "helper" methods for common "tool questions" (should we optimize, are we testing, etc.)  Ideally, custom configurations could indicate their own values for these questions, although that's outside the scope of this patch.

Doc PR to follow.

atbin is a proposed binary interchange format for atbuild and the
broader AT ecosystem.

atbuild has a weakly standardized input format: `build.atpkg`.  But what
does atbuild, um, actually build?  What's the output format?

There is no weak standard here, or even a convention.  It may build an
executable, a static library, or a dynamic one, or even a framework; it
may emit swiftmodule and swiftdoc files, or not.  A modulemap may or may
not be part of the build products and clients may or may not need it in
their search paths.

The uncertainty here complicates interoperability.  atpm should download
binary libraries, but what actually lives in that tarball?  Some random
dylibs we found in `.atllbuild/products`?

How do we build libraries for "fat" archs (like iOS, with 4 sub-archs)?
How would we write an `atinstall` tool that installs/updates
atpm/atbuild (or makes homebrew packages out of them)?

atbin proposes to answer these questions, providing a simple, portable,
hackable binary interchange format for all platforms and all AT
projects.

An `atbin` is a folder that ends in `.atbin`.  It contains, at least, a
`built.atpkg` file.

`built.atpkg` is clojure syntax identical to the more familiar
`build.atpkg`.  You can include tasks or w/e in there, although why
would want to, I'm not totally sure (this is Anarchy Tools though,
knock yourself out.)  However, the important bit is this:

```clojure
(package
    :name "foo"
    :payload "libFoo.a"
    :platforms ["ios-x86_64" "ios-i386"]
    :type "static-library"
)
```

(Other fields could also be present, this is not a complete enumeration)

This `.atbin` folder will then contain:

* `libFoo.a`, a fat library for the indicated platforms
* (optional) a `ios-x86_64.swiftmodule` and `ios-i386.swiftmodule` file
* (optional) a `ios-x86_64.swiftdoc` and `ios-i386.swiftdoc` file
* (optional) a `module.modulemap` file

You can, of course, build an `.atbin` by hand from existing binaries you
found lying around your disk.  And we may eventually ship an `atbin`
packager for Xcode or SwiftPM projects.

However more practically, we introduce a new tool, `packageatbin`, which
packages an `atbin` payload from atllbuild.

```clojure
:package {
   :tool "packageatbin"

   ;; Generate a mypayload.atbin
   :name "mypayload"

   ;; When used with the new --platform ios, will build a fat binary for all iOS platforms.
   ;; Alternatively specific platforms can be listed here
   :platforms ["all"]

   ;; The atllbuild task to package.
   ;; Special logic will re-run this task for each platform and merge the resulting output.
   :atllbuild-task "myatllbuildtask"
}
```

The obvious application is as an interchange format for prebuilt
`atllbuild` dependencies.  Presently, `atllbuild` can link with the
output of any dependent atllbuild task, but if a library wasn't produced
by a dependent task as part of the current build (but was say produced
on a different computer a month ago) there's no "obvious" way to link to
it.  This PR does not actually include any of that tooling, but it would
be a straightforward extension of this work.

An second application is the building of fat files.  Currently, there is
no mechanism to build a "fat" library or binary in atbuild, or even to
specify that we want one.  Under this PR, we can do it.

A third application is a distribution format for executables.  If an
`.atbin` contains an `executable`, `atpm` (or hypothetical `atinstall`)
could install/update/administrate that executable similar to `homebrew`
or `apt`, and keep all your buildtools (or other random crap) up to
date.  We would need to extend this with version fields and whatnot, but
again, it's straightforward.

An fourth application, and my real motivation, is as an intermediate
binary representation.  An `atbin` can be "downcast" with another tool
to a platform-native format like `deb`, `bottle`, or `Framework`.  This
would allow us to cut debs, rpms, and framework releases with
appropriate AT tools.

One alternative is to adopt an existing "standard", like Framework, for
this purpose.  Indeed, `atbuild` currently produces frameworks on OSX.

There are some complexities of extending frameworks to this case.  For
one, the Framework implementation is warty and involves a lot of
symlinks and things like codesigning.  We don't currently maintain that
code for Linux hosts, nor is the standard especially sensible for Linux,
as it relies on plists and choices basically unpopular on that platform.

For another, frameworks are not really built to house static library or
executable payloads, which are important to atbuild.  There are air-
quote "obvious" ways to extend to nontraditional payloads, but IMO this
is more confusing than it is helpful.  An explicit step to "cast down"
your atbin to a framework lets us check that your framework will
actually make sense to the likes of Xcode.

For a third, it's unclear what platform some random Framework is built
for, and what architectures it supports.  You can find out by scripting
out to platform-specific tools, but it's not portable.

Another alternative is to support multiple payloads/libraries in a
single atbin, "one atbin to rule them all".  However I don't see what we
accomplish there that we don't accomplish with multiple atbins, except
specification complexity.  So let's not do that, at least not initially.

`packageatbin` is included in core primarily because it needs tight,
source-level integration with atllbuild.  In addition to peeking at the
atllbuild options it needs to run the atllbuild task several times in
order to produce fat binaries, which means it has to work around the
usual dependency pruning logic.  For that reason it can't be sensibly
implemented via the current custom tool API.

We've broken this a couple of times, introduce test coverage so we stop
breaking it.

Related to #36

Presently, we tend to enable platform-specific config with an overlay.
There are a variety of problems identified with this approach:

1.  There is no convention for which overlay to use for platform-
specific config.  This complicates the ecosystem.

2.  In general, a program is always compiled "for some platform" but in
practice a user may forget the necessary overlay.  `require-overlays`
can catch this misconfig, but A) it has to be opted into in the atpkg,
and B) there is no good way to default to the running platform, which is
the sane behavior.

3.  Currently, tools tend to conditionally-compile platform-specific
code.  The effect of this is to complicate bootstrapping, as a Linux
binary *cannot* perform some OSX behavior (and vice versa) because the
necessary code is not present in the executable.

4.  This is not scaleable to cross-compilation (iOS for example is
Coming Soon™ but can't be supported in this architecture)

To work around these problems, we introduce a `Platform` enum, to
replace `PlatformPaths`.  `Platform` is a runtime technology, allowing
for a Linux binary to reason about what the behavior would be on OSX
etc.

Internally, we have three "platforms":

* `hostPlatform`, the platform on which `atbuild` is currently executing
* `targetPlatform`, the platform for which we are compiling. By default
   this is the `hostPlatform`
* `buildPlatform`, the platform where `swift-build-tool` will run.
   This is usually the `hostPlatform`, but if we are bootstrapping it
   is the `targetPlatform` instead.

The user can override the `targetPlatform` by the use of `--platform
foo`.  `linux` and `osx` are supported.  `mac` is supported as an alias
of `osx`.

The primary effect of a platform is to scope tool-specific behavior
(e.g., target=OSX uses the OSX SDK, host=Linux uses a linux path for the
toolchain, etc).

In addition to the tool-specific behavior, we enable overlays for the
target platform:

* `atbuild.platform.linux` on Linux
* `atbuild.platform.osx` and `atbuild.platform.mac` on OSX

This allows packages to reliably perform per-platform configuration in
the overlays.  Critically, some platform overlay is reliably active, so
users in most cases will not have to `--use-overlay` to get proper
platform behavior.

DEPRECATION WARNING: We believe the `swiftc-path` key is no longer
required, as the functionality used can be achieved either by
`--toolchain` or `--platform`.  Therefore, I'm adding a warning to users
that we intend to remove it and to try these features instead.

We need to put out a release with these features (and the warning)
before I'm happy to remove it.  In particular, we use it inside
atbuild/atpkg, and removing it immediately would break bootstrapping, so
let's give it a little time before we tear it out.  We should remove it
from the documentation though.

This allows atbuild to use a different toolchain other than the one we
use to develop atbuild (weekly snapshot).

In particular, this allows you to use "released swift" "xcode swift" or
any other kind of 2.2 Swift.

Documentation PR to follow.

Edited README to discuss atbuild options.

Resolves #58 to my satisfaction.

Update to swift-DEVELOPMENT-SNAPSHOT-2016-03-24-a

Update for atpkg:mute_warnings, which changes the package constructor

We silence certain redundant warnings that are probably useless.

This allows the building of files other than build.atpkg

This was fixed in snapshot

swift-DEVELOPMENT-SNAPSHOT-2016-01-25-a