Add support for bit fields

[kennykerr]

Rust lacks support at the moment: rust-lang/rfcs#314

Metadata now provides this information: microsoft/win32metadata#1392

[mominshaikhdevs]

I don't see the point of this issue. Since, rust the language itself doesn't support that. I believe this can be reopened and worked on once the lang itself supports it.

[kennykerr]

We could add get/set methods.

[workingjubilee]

We do indeed effectively farm out this responsibility to crates, essentially so that those crates can implement the getters/setters that strike their fancy. I believe rust-bindgen provides some such support.