Future Handles

A rust library to complete futures via a remote handle.

```rust async fn func() -> Option { let (future, handle) = unsync::create();

 func_with_callback(|| {
     handle.complete(1);
 });

 match future.await {
     // The callback was invoked and the result set via the handle.
     Ok(res) => Some(res),
     // The callback was never invoked, but the handle has been dropped.
     Err(_) => None
 }

} ```