CCSet: implement find_last_map using find_last

find_last exists since ocaml 4.05, using it for find_map avoid the
linear time behavior on ocaml >= 4.05

src/core/CCSet.ml

@@ -151,19 +151,19 @@ module Make (O : Map.OrderedType) = struct
     | None -> raise Not_found
     | Some x -> x
 
-  (* linear time, must traverse the whole set… *)
+  include S
+
+  (* Use find_last which is linear time on OCaml < 4.05 *)
   let find_last_map f m =
     let res = ref None in
-    S.iter
+    let _ = S.find_last_opt
       (fun x ->
         match f x with
-        | None -> ()
+        | None -> false
-        | Some y -> res := Some y)
+        | Some y -> res := Some y; true)
-      m;
+      m in
     !res
 
-  include S
-
   let add_seq seq set =
     let set = ref set in
     Seq.iter (fun x -> set := add x !set) seq;