ocaml: phase 6 Stack + Queue modules (+5 tests, 430 total)

Stack: ref-holding-list LIFO. push/pop/top/length/is_empty/clear.
Queue: two-list (front, back) amortised O(1) queue. push/pop/length/
is_empty/clear. Both in OCaml syntax in runtime.sx.

lib/ocaml/runtime.sx

@@ -366,6 +366,48 @@
      let printf fmt = print_string fmt
    end ;;
 
+   module Stack = struct
+     let create () = ref []
+     let push x s = s := x :: !s
+     let pop s =
+       match !s with
+       | [] -> failwith \"Stack.pop: empty\"
+       | h :: t -> s := t ; h
+     let top s =
+       match !s with
+       | [] -> failwith \"Stack.top: empty\"
+       | h :: _ -> h
+     let is_empty s = !s = []
+     let length s = List.length !s
+     let clear s = s := []
+   end ;;
+
+   module Queue = struct
+     (* Simple two-list amortized queue: (front, back). pop drains
+        front; refill from rev back when front is empty. *)
+     let create () = ref ([], [])
+     let push x q =
+       let p = !q in
+       q := (List.append [] (match p with (f, b) -> f), x :: (match p with (_, b) -> b))
+     let length q =
+       let p = !q in
+       let f = match p with (f, _) -> f in
+       let b = match p with (_, b) -> b in
+       List.length f + List.length b
+     let is_empty q = length q = 0
+     let pop q =
+       let p = !q in
+       let f = match p with (f, _) -> f in
+       let b = match p with (_, b) -> b in
+       match f with
+       | h :: t -> q := (t, b) ; h
+       | [] ->
+         (match List.rev b with
+          | [] -> failwith \"Queue.pop: empty\"
+          | h :: t -> q := (t, []) ; h)
+     let clear q = q := ([], [])
+   end ;;
+
    module Buffer = struct
      let create _ = ref []
      let add_string b s = b := s :: !b