Crate num_format_windows

A helper crate for num-format that only exists because num-format on Windows requires bindgen as a build dependency, which, if not broken out into a separate crate, would prevent num-format from being built in a no_std environment (build dependendies cannot currently be made contingent on the compilation target).

Said differently, don’t use this crate directly. It’s just a work-around for https://github.com/rust-lang/rust-bindgen/issues/1439.