impl BufMut for ReadBuf (?)

[coolreader18]

I came across this bit of code in hyper:

                let copy_len = cmp::min(prefix.len(), buf.remaining());
                // TODO: There should be a way to do following two lines cleaner...
                buf.put_slice(&prefix[..copy_len]);
                prefix.advance(copy_len);

(where prefix: Bytes and buf: &mut ReadBuf<'_>), and realized that this could be made (somewhat) simpler if ReadBuf implemented bytes::BufMut:

                let prefix_buf = (&mut prefix).take(buf.remaining_mut());
                buf.put(prefix_buf);

Is there an API reason ReadBuf doesn't implement BufMut under cfg_io_util! {}? I saw #3884 but that seems to be the opposite direction and is also a helper method, rather than a cfg()'d impl. Not sure if tokio would wanna do those, since it could maybe be more of an API hazard. An alternative is to have a like fn as_buf_mut(&mut self) -> ReadBufBufMut helper, either in tokio or in tokio-util. IMO this is core enough that it's not necessary to stick it intokio-util, since tokio depends on BufMut already and it's a very simple mapping from ReadBuf -> BufMut (like, 15 LOC).

[Darksonn]

Sure, sounds good to me. I'll accept a PR for this if anyone submits it.