<Arc<T>>::make_mut returns a &mut T, such an API is doable for Bytes too
and thus we should reserve Bytes::make_mut for that.
Furthermore, it would be helpful to use From<Bytes> as a trait bound
in some cases with other traits such as Hyper's body trait, where Hyper
gives you Bytes values.
Finally, making it impl From<Bytes> for BytesMut means the API is more
easily discoverable as it appears on both Bytes and BytesMut.
