Add String.split_on_char to compatibility layer

Compat.ml.in

@@ -108,6 +108,17 @@
 403:    init (length s) (fun i -> Char.lowercase_ascii (unsafe_get s i))
 403:  let uppercase_ascii s =
 403:    init (length s) (fun i -> Char.uppercase_ascii (unsafe_get s i))
+404:
+404:  let split_on_char sep s =
+404:    let r = ref [] in
+404:    let j = ref (length s) in
+404:    for i = length s - 1 downto 0 do
+404:      if unsafe_get s i = sep then begin
+404:        r := sub s (i + 1) (!j - i - 1) :: !r;
+404:        j := i
+404:      end
+404:    done;
+404:    sub s 0 !j :: !r
 403:end
 
 403:module Uchar = struct