This is the javascript equivalent of Async_unix.Scheduler, ie a reimplementation of the async scheduler for javascript (at least the part of it that makes sense).
val sleep : float ‑> unit Async_kernel.Deferred.tsleep d is a deferred which becomes determined in d seconds.
val yield : unit ‑> unit Async_kernel.Deferred.tyield () returns a deferred that becomes determined after the current cycle
completes.