Experiment with using windows-rs for bindings

[TyPR124]

See https://crates.io/crates/windows

Will probably have separate features for winapi and windows so user can choose which library to build with.

If using windows works well, may consider switching default to this for an eventual 1.0 release.

For the sake of consistency, will leave winapi as the default at least until a breaking-change release.

If the default does change, it should be documented in RELEASES