ocaml: phase 6 Array.sort/sub/append/exists/for_all/mem (+5 tests, 520 total)

Eight new Array functions, all in OCaml syntax inside runtime.sx,
delegating to the corresponding List operation on the cell's
underlying list:

  sort cmp a    -> a := List.sort cmp !a    (* mutates the cell *)
  stable_sort   = sort
  fast_sort     = sort
  append a b    -> ref (List.append !a !b)
  sub a pos n   -> ref (take n (drop pos !a))
  exists p      -> List.exists p !a
  for_all p     -> List.for_all p !a
  mem x a       -> List.mem x !a

Round-trip:
  let a = Array.of_list [3;1;4;1;5;9;2;6] in
  Array.sort compare a;
  Array.to_list a    = [1;1;2;3;4;5;6;9]

lib/ocaml/runtime.sx

@@ -560,6 +560,31 @@
      let to_list a = !a
      let of_list xs = ref xs
      let copy a = ref !a
+     let sort cmp a = a := List.sort cmp !a
+     let stable_sort = sort
+     let fast_sort = sort
+
+     let append a b = ref (List.append !a !b)
+
+     let sub a pos n =
+       let rec take xs k =
+         if k = 0 then []
+         else match xs with
+              | [] -> []
+              | h :: t -> h :: take t (k - 1)
+       in
+       let rec drop xs k =
+         if k = 0 then xs
+         else match xs with
+              | [] -> []
+              | _ :: t -> drop t (k - 1)
+       in
+       ref (take (drop !a pos) n)
+
+     let exists p a = List.exists p !a
+     let for_all p a = List.for_all p !a
+     let mem x a = List.mem x !a
+
      let blit src si dst di n =
        for k = 0 to n - 1 do
          set dst (di + k) (get src (si + k))