ocaml: phase 6 String.iter/iteri/fold_left/fold_right/to_seq/of_seq (+3 tests, 501 total)

Six new String functions, all in OCaml syntax inside runtime.sx:

  iter      : index-walk with side-effecting f
  iteri     : iter with index
  fold_left : thread accumulator left-to-right
  fold_right: thread accumulator right-to-left
  to_seq    : return a char list (lazy in real OCaml; eager here)
  of_seq    : concat a char list back to a string

Round-trip:
  String.of_seq (List.rev (String.to_seq "hello"))   = "olleh"

Note: real OCaml's Seq is lazy. We return a plain list because the
existing stdlib already provides exhaustive list operations and we
don't yet have lazy sequences. If a baseline needs Seq.unfold or
similar, we'll graduate to a proper Seq module then.

lib/ocaml/runtime.sx

@@ -374,6 +374,43 @@
          else aux (i + 1) (acc ^ f (_string_get s i))
        in
        aux 0 \"\"
+     let iter f s =
+       let rec aux i =
+         if i >= _string_length s then ()
+         else (f (_string_get s i); aux (i + 1))
+       in
+       aux 0
+     let iteri f s =
+       let rec aux i =
+         if i >= _string_length s then ()
+         else (f i (_string_get s i); aux (i + 1))
+       in
+       aux 0
+     let fold_left f init s =
+       let rec aux i acc =
+         if i >= _string_length s then acc
+         else aux (i + 1) (f acc (_string_get s i))
+       in
+       aux 0 init
+     let fold_right f s init =
+       let rec aux i acc =
+         if i < 0 then acc
+         else aux (i - 1) (f (_string_get s i) acc)
+       in
+       aux (_string_length s - 1) init
+     let to_seq s =
+       let rec aux i =
+         if i >= _string_length s then []
+         else _string_get s i :: aux (i + 1)
+       in
+       aux 0
+     let of_seq xs =
+       let rec aux ys acc =
+         match ys with
+         | [] -> acc
+         | h :: t -> aux t (acc ^ h)
+       in
+       aux xs \"\"
    end ;;
 
    module Bytes = struct