Use raise_notrace instead of raise to implement Hashtbl.replace.

This significantly improves the benchmark below:

  let ht = Hashtbl.create 100 in
  for x = 1 to 2500000 do
    Hashtbl.replace ht x ();
    Hashtbl.remove ht x
  done

(about 20% faster, still not as good as #328, which is about 35% faster
than trunk on this one)

In addition to the speedup, using raise_notrace should avoid destroying
the current stack trace when Hashtbl.replace is used in an exception
handler.

stdlib/hashtbl.ml

@@ -158,7 +158,7 @@ let find_all h key =
 let replace h key info =
   let rec replace_bucket = function
     | Empty ->
-        raise Not_found
+        raise_notrace Not_found
     | Cons(k, i, next) ->
         if compare k key = 0
         then Cons(key, info, next)
@@ -350,7 +350,7 @@ module MakeSeeded(H: SeededHashedType): (SeededS with type key = H.t) =
     let replace h key info =
       let rec replace_bucket = function
         | Empty ->
-            raise Not_found
+            raise_notrace Not_found
         | Cons(k, i, next) ->
             if H.equal k key
             then Cons(key, info, next)