Main thread.
This is evolved from Moonpool.Immediate_runner, but unlike it, this API assumes you run it in a thread (possibly the main thread) which will block until the initial computation is done.
This means it's reasonable to use Main.main (fun () -> do_everything) at the beginning of the program. Other Moonpool pools can be created for background tasks, etc. to do the heavy lifting, and the main thread (inside this immediate runner) can coordinate tasks via Fiber.await.
Aside from the fact that this blocks the caller thread, it is fairly similar to Background_thread in that there's a single worker to process tasks/fibers.
This handles effects, including the ones in Fiber.
val main : (Moonpool.Runner.t -> 'a) -> 'amain f runs f() in a scope that handles effects, including Fiber.await.
This scope can run background tasks as well, in a cooperative fashion.