Fix Windows cross-compilation and add ?Sized to use of Format trait

[anacrolix]

By using windows-sys we can entirely remove the need for bindgen, and Windows.h enabling cross compilation to Windows.

Additionally we relax the use of the Format trait to be ?Sized where possible, allowing to use fallback Format implementations through Box/dyn.

Please let me know if I missed anything, or you want it broken up more or something.