Copy data between readers and writers more carefully, with Nagle's algorithm? and auto-flushing?

The functionality in this function/future seem generally useful for implementing something kinda like sendfile/splice with AsyncRead and AsyncWrite; I'd suggest we make a new crate that provides this functionality, so both projects can use it. (And so can anybody else who wants it.)

This work is done with !3416 (merged), but this ticket eventually became a much more thoughtful discussion about what exactly we want to do with copied data, with implications for tor-proto design. Therefore, I'm going to keep it open until we extract whatever future changes we want.