Use Waker::will_wake()

[daxpedda]

I was wondering if there is any interest in adding this optimization, basically avoiding cloning the Waker if it isn't necessary.

Happy to make a PR.

[notgull]

Looking through the history of the original AtomicWaker implementation, it looks like it originally had a test like this, but it was removed later on.

I'm not familiar with futures' internals, but from a brief glance you could probably just add the check here.