breaking: remove Immediate_runner

it never really supported all that a runner should (effects, scheduling
other tasks, etc.)

src/core/immediate_runner.ml

@@ -1,20 +0,0 @@
-open Types_
-include Runner
-
-let run_async_ ~ls:cur_ls f =
-  TLS.get k_cur_storage := Some cur_ls;
-  try
-    let x = f () in
-    TLS.get k_cur_storage := None;
-    x
-  with e ->
-    let bt = Printexc.get_raw_backtrace () in
-    TLS.get k_cur_storage := None;
-    Printexc.raise_with_backtrace e bt
-
-let runner : t =
-  Runner.For_runner_implementors.create
-    ~size:(fun () -> 0)
-    ~num_tasks:(fun () -> 0)
-    ~shutdown:(fun ~wait:_ () -> ())
-    ~run_async:run_async_ ()

src/core/immediate_runner.mli

@@ -1,23 +0,0 @@
-(** Runner that runs tasks immediately in the caller thread.
-
-    Whenever a task is submitted to this runner via [Runner.run_async r task],
-    the task is run immediately in the caller thread as [task()].
-    There are no background threads, no resource, this is just a trivial
-    implementation of the interface.
-
-    This can be useful when an implementation needs a runner, but there isn't
-    enough work to justify starting an actual full thread pool.
-
-    Another situation is when threads cannot be used at all (e.g. because you
-    plan to call [Unix.fork] later).
-
-    {b NOTE}: this does not handle the [Suspend] effect, so [await], fork-join,
-    etc. will {b NOT} work on this runner.
-
-    @since 0.5
-*)
-
-include module type of Runner
-
-val runner : t
-(** The trivial runner that actually runs tasks at the calling point. *)

src/core/moonpool.ml

@@ -25,7 +25,7 @@ module Exn_bt = Exn_bt
 module Fifo_pool = Fifo_pool
 module Fut = Fut
 module Lock = Lock
-module Immediate_runner = Immediate_runner
+module Immediate_runner = struct end
 module Runner = Runner
 module Task_local_storage = Task_local_storage
 module Thread_local_storage = Thread_local_storage_

src/core/moonpool.mli

@@ -13,7 +13,13 @@ module Ws_pool = Ws_pool
 module Fifo_pool = Fifo_pool
 module Background_thread = Background_thread
 module Runner = Runner
-module Immediate_runner = Immediate_runner
+
+module Immediate_runner : sig end
+[@@deprecated "use Moonpool_fib.Main"]
+(** Runner that runs tasks in the caller thread.
+
+    This is removed since NEXT_RELEASE, and replaced by {!Moonpool_fib.Main}. *)
+
 module Exn_bt = Exn_bt
 
 exception Shutdown