Introduce new bindgen crate

[kennykerr]

• The new windows-bindgen crate replaces the existing gen crate. The latter is still used by the build and generate macros but those will be removed soon. Projects should simply use the bindgen crate directly.
• The bindgen crate is now used to generate the windows and windows-sys crates as well as the internal bindings.
• Code gen across windows and windows-sys is now much more closely aligned, making code maintenance much more manageable and for dependents switching between the two dependencies a lot easier.
• Notably, the Win32 portion of the bindings in the windows crate now behave a lot more like those in the windows-sys crate with fewer unnecessary type system adornments.
• The bindgen crate does not rely on the derive macro, producing code that compiles faster.
• The Debug trait is no longer implemented as it requires a lot of code gen and negatively impacts compile time.

The next step is to introduce a trait-based replacement for the implement macro and then the gen crate along with the build and generate macros will be removed.