Implement Borrow<QueueId> for QueuedWaker

[espindola]

This allows using self.queue.remove(&id) directly, which then allows other simplifications.

[espindola]

The "other simplifications" are

• No need to implement PartialEq, Ord, and PartialOrd for QueuedWaker
• Avoid a call to unwrap in Waitlist::pop