add stream::interval

[yoshuawuyts]

Adds stream::interval as "unstable", implementing #18 (comment). Had to incorporate #275 for this unfortunately also. This integrates the now defunct futures_timer::Interval code into async_std, building only on the Delay primitive. Thanks!

Closes #275.

cc/ @shaikh-raj

Example

use std::time::Duration;
use futures::prelude::*;

let interval = Interval::new(Duration::from_secs(4));
while let Some(_) = interval.next().await {
    println!("prints every four seconds"));
}

Screenshot