Synthesize swift interface for existing logic

[jerrymarino]

It is often hard and unapproachable to reason about the output of XCBBuildService and translate that into a higher level language. The other approach to a custom message pack implementation and related code is to generate an interface and call existing primitives in provided build system frameworks.

Now that the Swift ABI has become more stable we may have better luck calling it via ABI assumptions. Finally, the existence of a stable ABI also doesn't guarantee that this a sustainable approach, or allow us to guarantee existing frameworks will be callable from a snythetic interface or different compiler version. After all, Xcode, and macOS require updates annually and are subject to change.