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

Rust makes vanishingly few guarantees about the layout of pointers to unsized types. To my knowledge, the only exception is that pointers to str have the same layout as pointers to [u8]. I propose extending this guarantee to OsStr as well as Path. OsStr's three implementations and Path all currently support this as a consequence of being repr(transparent) newtypes of either str or [u8], but there are comments (not doc comments!) above the definitions of OsStr and Path stating that this "is considered an implementation detail and must not be relied upon".

The primary use case for this guarantee is casting/transmutation: it is easy enough to convert between &str or &[u8] and OsStr or Path, but the same cannot be said for conversions between &[&str] or &[&[u8]] and &[&OsStr] or &[&Path]. Given that OsStr and Path are by all accounts intended to be newtypes that model strings with constraints somewhere between those of &str and &[u8], I think it makes sense to provide layout guarantees in turn.