Pre-RFC: Guarantee that pointers to `OsStr` has the same layout as pointers to `str`/`[u8]`

May I ask why not? I discussed the proposal with some folks in the RPLC Discord server, and the only plausible issue that came up was if a future implementation wanted to store locale information in the pointer metadata, but given that every str must be an OsStr, I'm not convinced that this would be a worthwhile endeavor.