Switch to xctool

Use [xctool](https://github.com/facebook/xctool) instead of `xcodebuild`, because dealing with the latter requires way too many hacks. Note that this means that `xctool` _must_ be installed on the build machine.

I took this opportunity to significantly clean up the code. It actually looks kinda nice, if you squint and view it from a distance.

Also, `bootstrap` now automatically runs `script/bootstrap` for any submodule that has it. So there's that.