TzString: add fold_left

lib_stdlib/tzString.ml

@@ -73,3 +73,8 @@ let mem_char s c =
   match String.index s c with
   | exception Not_found -> false
   | _ -> true
+
+let fold_left f init s =
+  let acc = ref init in
+  String.iter (fun c -> acc := f !acc c) s ;
+  !acc

lib_stdlib/tzString.mli

@@ -29,3 +29,6 @@ val common_prefix: string -> string -> int
 
 (** Test whether a string contains a given character *)
 val mem_char: string -> char -> bool
+
+(** Functional iteration over the characters of a string from first to last *)
+val fold_left : ('a -> char -> 'a) -> 'a -> string -> 'a