Metadata processing improvements

[kennykerr]

Updates include:

• .winmd parser now more accurately captures generic type information for lossless roundtripping
• .winmd writer now supports generics
• .rdl generation and parsing now support generics
• More of the Rust-specific metadata transformation has moved to riddle

To this day, the Windows Runtime depends on a small .il file buried in the OS repo that needs to be parsed with ILASM in order to bootstrap the type system since no other tool can generate a .winmd file with generic types. This is why WinRT APIs have never been able to define their own generic types. Well now, riddle is able to generate a .winmd with generic type parameters and thereby bootstrap WinRT with no intermediate tools or dependencies.

Naturally, Win32 and WinRT metadata are supported seamlessly by the same tool.