Code gen overhaul

[kennykerr]

Notable changes:

• Win32 unions and nested structs are now generated correctly (Add support for C-style unions #417 Allow access to the Offset fields of the OVERLAPPED structure #569 Support for ADSVALUE #544 Simplify use of VARIANT  #539 Correct use of data types? #536)
• Improved support for many existing Win32 APIs and type signatures
• Support for convertible parameters on Win32 functions and COM methods (Methods consuming com objects cause leaks  #548)
• Support for well-known types like BOOL, BSTR, PWSTR, and PSTR (Add and use common type aliases #527 Support wide strings #484)
• Much smaller code gen, leading to faster build times
• Updated metadata providing many fixes or improvements to Win32 APIs
• Simpler internal metadata representation to support a broader set of type systems more naturally
• Many more improvements and fixes (Build macro should not have "special" logic for building Windows crate #568 Blittable structs don't need a distinct ABI struct surrogate #533 Trasition WinRT internals to use Win32 metadata #494 Remap common types to Windows.Foundation.Numerics #456 IAsync{Action,Operation}{WithProgress,} are not Send, so their impl Futures cannot be send across threads. #375)

There is also much more yet to do, but this overhaul makes it a lot easier to tackle those improvements incrementally and more quickly.

Regarding the smaller code gen, compiling the monstrous windows-docs-rs project sees quite an improvement...

Before:

• windows.rs: 338 MB
• build: 26 minutes

After:

• windows.rs: 177 MB
• build: 16 minutes

While we continue to need the Rust compiler to get faster, generating less code is also helpful. 😉