Create the concept of 'owned data' in upb/rust as a generalization of the upb.rs SerializedData (which is a arena + data for arbitrary types, both thin and wide ref types), use that for the wire parse/serialize path.

Create the concept of 'owned data' in upb/rust as a generalization of the upb.rs SerializedData (which is a arena + data for arbitrary types, both thin and wide ref types), use that for the wire parse/serialize path. #910