Add an example using a limiter

This demonstrates a layout problem where the fibers form a staircase.

Author: talex5

examples/limiter/dune

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+(executable
+  (name main)
+  (libraries eio.mock))

examples/limiter/main.ml

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+(* This runs jobs 4 at a time. It's useful to check eio-trace's layout algorithm. *)
+
+open Eio.Std
+
+let lock = Eio.Mutex.create ()
+
+let main () =
+  List.init 15 Fun.id
+  |> Fiber.List.iter ~max_fibers:4 (fun i ->
+      if i = 4 then Fiber.both Fiber.yield Fiber.yield;
+      Eio.Mutex.use_ro lock Fiber.yield)
+
+let () =
+  Eio_mock.Backend.run main