Use windows-bindgen crate to limit downloads/version bumps

[djc]

The windows project now exposes a crate that lets us build the bindings we need without the need to depend on all of them (with large downloads and regular version bumps). Would be nice to have.

[Ralith]

Hmm, wouldn't that compromise sharing with other crates? I guess we need very few definitions so maybe that's fine.

[djc]

Yeah, kind of the point is that most crates using the windows-sys stuff only need a tiny fraction of the bindings, so distributing everything to everyone is kind of pointless. Duplicating thin wrappers governed through a central code generator seems like a superior solution to me.