common/mlstdutils: Drop our implementations of functions now in OCaml 4.01.

We reimplemented some functions which can now be found in the OCaml
stdlib since 4.01 (or earlier).  The functions I have dropped are:

 - String.map
 - |>
 - iteri  (replaced by List.iteri)
 - mapi   (replaced by List.mapi)

Note that our definition of iteri was slightly wrong: the type of the
function parameter was too wide, allowing (int -> 'a -> 'b) instead of
(int -> 'a -> unit).

I also added this new function to the Std_utils.String module as an
export from stdlib String:

 - String.iteri

Thanks: Pino Toscano

generator/java.ml

@@ -330,7 +330,7 @@ public class GuestFS {
         pr "    /* Unpack optional args. */\n";
         pr "    Object _optobj;\n";
         pr "    long _optargs_bitmask = 0;\n";
-        iteri (
+        List.iteri (
           fun i argt ->
             let t, boxed_t, convert, n, default =
               match argt with