Returns true iff already signaled. Never resets the signal.
Detailed Description
A single-producer, single-consumer event.
Must not be awaited or signaled multiple times in the same waiting session.
The main advantage of SingleUseEvent over SingleConsumerEvent is that the waiting coroutine is allowed to immediately destroy the SingleUseEvent after waking up; it will not stop a concurrent Send from completing correctly.
Timeouts and cancellations are not supported. Only a concurrent call to Send can wake up the waiter. This is necessary for the waiter not to destroy the SingleUseEvent unexpectedly for the sender.