Fix bytes module

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/stdune/bytes.ml

@@ -1,7 +1,9 @@
 (* [blit_string] was forgotten from the labeled version in OCaml
    4.02—4.04. *)
-[@@@ocaml.warning "-32"]
+include StdLabels.Bytes
+
 let blit_string ~src ~src_pos ~dst ~dst_pos ~len =
   Caml.Bytes.blit_string src src_pos dst dst_pos len
 
-include StdLabels.Bytes
+let sub_string dst ~pos ~len =
+  Caml.Bytes.sub_string dst pos len

src/stdune/bytes.mli

@@ -7,3 +7,9 @@ val blit_string
   -> dst_pos:int
   -> len:int
   -> unit
+
+val sub_string
+  :  t
+  -> pos:int
+  -> len:int
+  -> string