Cancels the awaitable. If the awaitable is being awaited, the awaiter receives a System.OperationCanceledException.
Note: Some methods returning an Awaitable also accept a CancellationToken, such as Awaitable.WaitForSecondsAsync and Awaitable.NextFrameAsync. For such methods, both cancellation models are equivalent: whether you call Awaitable.Cancel directly or trigger the supplied CancellationToken, the awaiter receives an OperationCanceledException.