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

I had a PR up for specifying that transmute was valid between &OsStr and &[u8] and libs-api, at the time, decided to instead make this done through explicit functions. See Allow limited access to OsStr bytes by epage · Pull Request #109698 · rust-lang/rust · GitHub