Support std::io::{Empty, Sink}

[vorner]

Add async support for the „stub“ helpers of std::io.

Some motivation can be found here: https://vorner.github.io/2019/09/15/play-with-new-async.html#switching-to-tokio ‒ I find these helpers quite useful sometimes.

[taiki-e]

I think you missed #1829.

[vorner]

Sorry, I must have missed it, yes. I was checking by the version available on docs.rs.