* 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.