Replace futures-util with optional futures-io dep

[Ralith]

A trivial amount of additional code lets us save another dep and be a bit more accessible to new users (see e.g. #1262).

[Ralith]

This also improves consistency with tokio primitives, e.g. mpsc::Receiver::recv.

[djc]

Should we use futures-io as the optional dep rather than futures-util? Otherwise this looks great!

[Ralith]

Good catch, I didn't know that crate existed! Fixed.