feat: add Moonpool_fib.main

this is a convenient entrypoint for programs that use fibers. The main
thread can thus await fibers that run in the background.

src/fib/main.ml

@@ -0,0 +1,16 @@
+exception Oh_no of Exn_bt.t
+
+let main (f : Runner.t -> 'a) : 'a =
+  let st = Fifo_pool.Private_.create_state ~threads:[| Thread.self () |] () in
+  let runner = Fifo_pool.Private_.runner_of_state st in
+  try
+    let fiber = Fiber.spawn_top ~on:runner (fun () -> f runner) in
+    Fiber.on_result fiber (fun _ -> Runner.shutdown_without_waiting runner);
+    (* run the main thread *)
+    Fifo_pool.Private_.run_thread st runner ~on_exn:(fun e bt ->
+        raise (Oh_no (Exn_bt.make e bt)));
+    match Fiber.peek fiber with
+    | Some (Ok x) -> x
+    | Some (Error ebt) -> Exn_bt.raise ebt
+    | None -> assert false
+  with Oh_no ebt -> Exn_bt.raise ebt

src/fib/main.mli

@@ -0,0 +1,25 @@
+(** 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}.
+
+    @since NEXT_RELEASE
+*)
+
+val main : (Moonpool.Runner.t -> 'a) -> 'a
+(** [main f] runs [f()] in a scope that handles effects, including {!Fiber.await}.
+
+    This scope can run background tasks as well, in a cooperative fashion. *)

src/fib/moonpool_fib.ml

@@ -3,4 +3,7 @@
 module Fiber = Fiber
 module Fls = Fls
 module Handle = Handle
+module Main = Main
 include Fiber
+
+let main = Main.main