Introduce windows-core crate

[kennykerr]

This update adds the windows-core crate that takes the foundational support in the windows crate's core module and splits it out into a dedicated crate so that it may be used without the hefty windows crate, if needed.

The windows-core crate also ditches the hand-rolled internal bindings and instead depends on standalone bindings generated by the windows-bindgen crate.

The windows-bindgen crate’s standalone code generation has been upgraded to support generating bindings for COM and WinRT APIs as well. The standalone_sys function will generate windows-sys style bindings while the standalone_win function will generate windows style bindings. The latter fully supports COM and WinRT types.

Developers using the windows-sys crate are unaffected by this update as it does not depend on the windows-core crate and continues to be the recommended approach for raw bindings to the C-style Windows APIs.

Developers using the windows crate are mostly unaffected by this update as a dependency on the windows-core crate is automatically added. However, if you depend on the component authoring support, you will need to add a dependency to the windows-core crate yourself since the windows-bindgen crate generates code that only depends on the windows-core crate to avoid a circular dependency.

This update further addresses feedback from #1720.