Use our own io::{Empy, Repeat, Sink}

[taiki-e]

If both futures::io ext traits and std::io traits are imported, method names may conflict, so use our own.

[yoshuawuyts]

We've had this problem in async-std as well, and having these be their own impls makes a lot of sense!

[taiki-e]

(btw, I'm considering adding our own Cursor in another PR. (It was a little big so I separated it from this PR))

[cramertj]

I have no particularly strong feels about this.