Add Future::delay

[yoshuawuyts]

Similar to #309 we should add a method to Future that delays execution for a Duration. This method should be marked as "unstable".

This is useful in some cases where you want to define a Future that has a delay, rather than combining task::sleep with an async block and another future.

Examples

use async_std::future;

let a = future::ready(1).delay(Duration::from_millis(200));
let b = future::ready(2).delay(Duration::from_millis(100));
let c = future::ready(3).delay(Duration::from_millis(300));

dbg!(future::join!(a, b, c).await);

This is simlilar to this, but less nice to write:

let a = async {
    task::sleep(Duration::from_millis(200).await;
    future::ready(1).await
};
let b = async {
    task::sleep(Duration::from_millis(100).await;
    future::ready(2).await
};
let c = async {
    task::sleep(Duration::from_millis(300).await;
    future::ready(3).await
};

dbg!(future::join!(a, b, c).await);