Await_kernel.YieldSourceAbstraction implementation of cooperatively yielding to the scheduler.
Operations that run long computations and might want to yield to a scheduler take a t that provides an implementation of yielding for them to use.
Internally, a Yield.t is just an Await.t, but code which only yields and does not block can take a Yield.t as a parameter to promise that it will not block.
Represents the capability to cooperatively yield to the scheduler.
of_sync sync is an implementation of yielding based on sync