Add the ability to build Xcode projects with Bazel

It'd be awesome to integrate `XCHammer` project generation with Bazel. This would allow remote caching of Xcode projects using the existing Bazel infrastructure, which could speed up workflows like rebasing master and branch switching.

Considerations:
- Running Aspects: With the ability to [run aspects](https://docs.bazel.build/versions/master/skylark/aspects.html) from a rule, this should be technically possible. It'd be nice if we could write a rule that uses the output of aspects as a input. It might be worth looking into `cquery` too.
- Run Bazel queries: we'd want to run the query as part of the rule as well.
- Xcode projects are portable across machines: paths, noop inputs, are the minimum.
- Multi project outputs: with N > 1 projects, it'd be ideal to output 1 Xcode project per Bazel target.