Pre-generate slices of the Windows API

[rylev]

Many are wondering if certain subsets of the API surface could be pre-generated and packaged as crates. This can avoid running the build script and proc-macros on fresh builds which can improve build times.

The questions are:

• Is this a good idea in the abstract?
• How should the APIs can partitioned into crates?
  • Should there be one win32 crate with subsections composed by features?
  • Or should each sub API (e.g., direct2d) be its own separate crate?

[rylev]

Duplicate of #432. Closing this one in favor of that.