Analyzing compile time indicates that most of the time building the windows crate is spent parsing function bodies. Those functions provide support for a more idiomatic developer experience. Even though those function bodies essentially compile away to nothing, the extra compile time makes adoption of the windows crate prohibitive in some cases. The windows-sys crate will simply have function declarations (externs), structs, and constants, and not include any function bodies. This is essentially the underlying ABI for the Windows API.

Today, the windows crate "inlines" this ABI and hides it from the developer. With the introduction of the windows-sys crate, the windows crate will instead defer to the windows-sys crate for those declarations. A developer can then choose to use the windows crate directly for convenience, safety, and productivity, or use the underlying windows-sys crate for no-compromise compile time when that matters most.

As always, all declarations will be generated from metadata. This PR just adds the basic scaffolding. Splitting out the ABI code gen is next.