get rid of compare_val in String.concat

stdlib/bytes.ml

@@ -113,7 +113,7 @@ let iter f a =
 let iteri f a =
   for i = 0 to length a - 1 do f i (unsafe_get a i) done
 
-let ensure_ge x y = if x >= y then x else invalid_arg "Bytes.concat"
+let ensure_ge (x:int) y = if x >= y then x else invalid_arg "Bytes.concat"
 
 let rec sum_lengths acc seplen = function
   | [] -> acc

stdlib/string.ml

@@ -50,7 +50,7 @@ let fill =
 let blit =
   B.blit_string
 
-let ensure_ge x y = if x >= y then x else invalid_arg "String.concat"
+let ensure_ge (x:int) y = if x >= y then x else invalid_arg "String.concat"
 
 let rec sum_lengths acc seplen = function
   | [] -> acc