972: Restructure USB crates r=Dirbaio a=Dirbaio
- Split driver from `embassy-usb` to a separate crate. This allows making breaking changes to `embassy-usb` without having to bump all the crates with driver impls, such as HALs.
- Merge classes into `embassy-usb`. Now that breaking changes to `embassy-usb` aren't that bad, having everything in a single crate is much easier.
Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>
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