[wish] function to construct a promise that is already fulfilled

[mbacarella]

If the task is small enough you may not want to dispatch it to the thread pool.

In those cases it would be nice if you could simply create a promise that's already fulfilled, so that code that runs await on it can work in either case.

Supposing we can have a Task.return that creates a fulfilled promise, here's a contrived example:

let work, worklen = get_work () in
let changes =
  (* skip dispatching to pool if workload is small *)
  if worklen < 3
  then List.map work ~f:(fun x -> Task.return pool (thunk_opt x))
  else List.map work ~f:(fun x -> Task.async pool (fun () -> thunk_opt x)))
in
List.fold_left changes ~init:[] ~f:(fun acc res ->
  match Task.await pool res with
  | None -> acc
  | Some y -> y :: acc)