Interval API

It would be great if Timer was not just a Future but also a Stream so that it could act as a "ticker" delivering events at regular intervals.

Proposed API:

impl Timer {
    fn interval(period: Duration) -> Timer;
    fn interval_at(start: Instant, period: Duration) -> Timer;

    fn set_interval(&mut self, period: Duration);
    fn set_interval_at(&mut self, start: Instant, period: Duration);
}

impl Stream for Timer {
    type Item = Instant;
}

Thoughts or suggestions?

Published v1.3.0