Add Future::flatten

[yoshuawuyts]

Similar to future-rs's Future::flatten method, this can sometimes come in useful.

However the only difference with it should be that we take IntoFuture as the trait bound, and call it under the hood. Because IntoFuture is implemented for Future this should work as expected.

Thanks!

Examples

let nested_future = async { async { 1 } };
let future = nested_future.flatten();
assert_eq!(future.await, 1);

[abhishekc-sharma]

I can take a stab at this

[skade]

@portgasd666 go ahead!

[k-nasa]

This seems to have been resolved! by #443