rust: Use Bytes instead of Vec<u8> in protobufs

[gasmith]

For bytes fields on protobufs, we should use Bytes instead of Vec<u8>. This enables zero-copy for buffers that are already Bytes of course, but also for types T: AsRef<[u8]> + Send + 'static (via Bytes::from_owner), such as you'd find for an Arc<Vec<u8>> or Mmap or other various memory views.

[bryfox]

Should also end up being useful for #220.