Consider adding extension traits for ergonomics

[shepmaster]

Instead of e.g.

let consumer = timer
    .timeout_stream(socket, Duration::from_secs(2))
    .filter(|&v| v == 2)

I think that preserving the chaining of the future or stream would look better:

let consumer = socket
    .timeout(&timer, Duration::from_secs(2))
    .filter(|&v| v == 2)

Note that this allows dropping the _stream from the name.