Switch to windows-sys on 0.4.x branch

[pitdicker]

#1003 (comment) mentioned we can switch from winapi to windows-sys on the 0.4.x branch now that our MSRV exceeds 1.48.

But I don't know if there are other concerns.

[pitdicker]

See #712 for the original PR to switch to windows-sys.

[djc]

I'm open to this -- an alternative approach which I think would be preferrable would be to use the windows-bindgen crate to generate more minimal API surface in an integration test instead of using windows-sys. The problem with windows-sys is that it adds lots of download size, so it's not a great fit for low-level crates like chrono.

[pitdicker]

Documentation on windows-bindgen:
https://github.com/microsoft/windows-rs/blob/0.48.0/docs/readme.md#windows-bindgen
https://kennykerr.ca/rust-getting-started/standalone-code-generation.html

It does come with a note:

Warning: Standalone code generation should only be used as a last resort for the most demanding scenarios. It is much simpler to use the windows-sys crate and let Cargo manage this dependency. This windows-sys crate provides raw bindings, is heavily tested and widely used, and should not meaningfully impact your build time.

I'll have a look at it.

[djc]

I think chrono would qualify as a fairly demanding scenario, and I think the warning is maybe a bit overblown.

See more context in microsoft/windows-rs#1720.