AsyncBufReader / AsyncBufWriter

[yoshuawuyts]

Similar to #1512, we've been missing an async equivalent of std's io::BufReader and io::BufWriter. This would be useful to reduce the amount of system calls, and speed up throughput.

[Nemo157]

I've experimented with this in embrio with trait AsyncBufRead, an async struct BufReader and an implementation of async fn read_until (the latter can definitely be simplified by providing async fn fill_buf() on top of fn poll_fill_buf() and actually using async fn syntax). These have definitely worked well, and it should be straightforward to port std-like variants into futures (these were a little different than the ones provided in std since embrio is alloc-less).

[taiki-e]

std::io::BufWriter implements Drop, and it is convenient. Do you think that async BufWriter can also implement it? (functions that call flush_buf (flushes the internal buffer) internally need to receive a context. into_inner also has the same problem.)

[Nemo157]

IMO anyone using an async writer should be calling close on it to ensure it's fully flushed and closed, dropping a writer without closing it is allowed to just abandon any outstanding writes (which is something that probably needs documenting on AsyncWrite). Having std::io::Write implementations set the precedent that dropping will flush outstanding writes and swallow errors is quite unfortunate though.