refactor: small optimization to Fiber.Ivar.{read,fill}

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

<!-- ps-id: ad3b62b7-f72f-46f7-ba70-c9d925a0330a -->

fiber/src/core.ml

@@ -217,8 +217,20 @@ module Ivar = struct
   type 'a t = 'a ivar
 
   let create () = { state = Empty }
-  let read t k = Read_ivar (t, k)
-  let fill t x k = Fill_ivar (t, x, k)
+
+  let read t k =
+    match t.state with
+    | Full x -> k x
+    | Empty_with_readers _ | Empty -> Read_ivar (t, k)
+  ;;
+
+  let fill t x k =
+    match t.state with
+    | Empty ->
+      t.state <- Full x;
+      k ()
+    | Full _ | Empty_with_readers _ -> Fill_ivar (t, x, k)
+  ;;
 
   let peek t k =
     k