Added a 'stack module' to be paired with calc.ml

generator/tests/calc.ml

+open Stack
+
 type expr =
   | Const [@f value] of int
   | Add [@f left, right] of expr * expr

generator/tests/stack.ml

+type 'a stack =
+  | Nil
+  | Cons [@f value, stack] of 'a * 'a stack
+
+let is_empty s = s === Nil
+
+let push x stack = Cons(x, stack)
+
+let pop stack = 
+  match stack with
+  | Cons (x, xs) -> x
+  | Nil -> stuck "Empty list"
+