Remove some `Reference` bounds

Currently, some `Ptr` APIs require `A: Reference` where likely `A: Aliasing` would be sufficient. In particular, these APIs merely need the ability to read from their referents, and are compatible with owned pointers. In particular:

- `Ptr::as_ref` (though this would require changing `self` to `&self` to avoid moving owned pointers)
- `Ptr::reborrow`
- `Ptr::try_into_valid`
- `Ptr::as_bytes`
- `Ptr::try_cast_into` / `try_cast_into_no_leftover`