Tighten BytesMut guarantees

[danburkert]

The docs for BufMut::bytes_mut specifically allow for the method to return an empty slice . This should be changed so that the method may only return a 0-length slice if the buffer is exhausted. Otherwise, it's extremely easy to trigger infinite loops (for instance, the current implementation of BufMut::put_slice). I believe the stdlib Write trait puts similar restrictions on 0-length writes for similar reasons.

Likewise, BufMut::bytes_vec_mut should not return a 0-length vector unless the buffer is exhausted, and probably shouldn't return 0-length IoVecs ever.