Add the ability to build Xcode projects with Bazel

[jerrymarino]

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 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.

[jerrymarino]

This was prototyped out in #190