Remove some Reference bounds

[joshlf]

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