Deprecate Fiber.fork_sub

Originally, you needed the parent switch to create the child one, but
that happens automatically now, so we don't need a special function for
this.

lib_eio/core/eio__core.mli

@@ -235,14 +235,7 @@ module Fiber : sig
       The calling fiber is placed at the head of the run queue, ahead of any previous items. *)
 
   val fork_sub : sw:Switch.t -> on_error:(exn -> unit) -> (Switch.t -> unit) -> unit
-  (** [fork_sub ~sw ~on_error fn] is like [fork], but it creates a new sub-switch for the fiber.
-
-      This means that you can cancel the child switch without cancelling the parent.
-      This is a convenience function for running {!Switch.run} inside a {!fork}.
-
-      @param on_error This is called if the fiber raises an exception.
-                      If it raises in turn, the parent switch is failed.
-                      It is not called if the parent [sw] itself is cancelled. *)
+  [@@deprecated "Use Fiber.fork and Switch.run separately instead"]
 
   val fork_promise : sw:Switch.t -> (unit -> 'a) -> 'a Promise.or_exn
   (** [fork_promise ~sw fn] schedules [fn ()] to run in a new fiber and returns a promise for its result.

tests/fiber.md

@@ -218,7 +218,7 @@ Exception: Stdlib.Exit.
 Exception: Failure "simulated error".
 ```
 
-# Fiber.fork
+# Fiber.fork_promise
 
 `Fiber.fork_promise ~sw` inherits the cancellation context from `sw`, not from the current fiber:
 

tests/switch.md

@@ -10,6 +10,13 @@ open Eio.Std
 let run (fn : Switch.t -> _) =
   Eio_mock.Backend.run @@ fun () ->
   Switch.run fn
+
+let fork_sub ~sw ~on_error fn = Fiber.fork ~sw (fun () ->
+    try Switch.run fn
+    with
+    | Eio.Cancel.Cancelled _ -> ()
+    | ex -> on_error ex
+  )
 ```
 
 # Test cases
@@ -200,8 +207,8 @@ Child switches are cancelled when the parent is cancelled, but `on_error` isn't
 # run (fun sw ->
       let p, _ = Promise.create () in
       let on_error ex = traceln "child: %s" (Printexc.to_string ex) in
-      Fiber.fork_sub ~sw ~on_error (fun sw -> traceln "Child 1"; Promise.await p);
-      Fiber.fork_sub ~sw ~on_error (fun sw -> traceln "Child 2"; Promise.await p);
+      fork_sub ~sw ~on_error (fun sw -> traceln "Child 1"; Promise.await p);
+      fork_sub ~sw ~on_error (fun sw -> traceln "Child 2"; Promise.await p);
       Switch.fail sw (Failure "Cancel parent")
     );;
 +Child 1
@@ -216,8 +223,8 @@ A child can fail independently of the parent:
       let p1, r1 = Promise.create () in
       let p2, r2 = Promise.create () in
       let on_error ex = traceln "child: %s" (Printexc.to_string ex) in
-      Fiber.fork_sub ~sw ~on_error (fun sw -> traceln "Child 1"; Promise.await_exn p1);
-      Fiber.fork_sub ~sw ~on_error (fun sw -> traceln "Child 2"; Promise.await_exn p2);
+      fork_sub ~sw ~on_error (fun sw -> traceln "Child 1"; Promise.await_exn p1);
+      fork_sub ~sw ~on_error (fun sw -> traceln "Child 2"; Promise.await_exn p2);
       Promise.resolve_error r1 (Failure "Child error");
       Promise.resolve_ok r2 ();
       Fiber.yield ();
@@ -237,7 +244,7 @@ A child can be cancelled independently of the parent:
       let p, _ = Promise.create () in
       let on_error ex = traceln "child: %s" (Printexc.to_string ex) in
       let child = ref None in
-      Fiber.fork_sub ~sw ~on_error (fun sw ->
+      fork_sub ~sw ~on_error (fun sw ->
           traceln "Child 1";
           child := Some sw;
           Promise.await ~sw p
@@ -258,7 +265,7 @@ A child error handler raises:
 # run (fun sw ->
       let p, r = Promise.create () in
       let on_error = raise in
-      Fiber.fork_sub ~sw ~on_error (fun sw -> traceln "Child"; Promise.await_exn p);
+      fork_sub ~sw ~on_error (fun sw -> traceln "Child"; Promise.await_exn p);
       Promise.resolve_error r (Failure "Child error escapes");
       Fiber.yield ();
       traceln "Not reached"
@@ -273,7 +280,7 @@ A child error handler deals with the exception:
 # run (fun sw ->
       let p, r = Promise.create () in
       let on_error = traceln "caught: %a" Fmt.exn in
-      Fiber.fork_sub ~sw ~on_error (fun sw -> traceln "Child"; Promise.await_exn p);
+      fork_sub ~sw ~on_error (fun sw -> traceln "Child"; Promise.await_exn p);
       Promise.resolve_error r (Failure "Child error is caught");
       Fiber.yield ();
       traceln "Still running"