ocaml: phase 1+5.1 type aliases + poly_stack baseline (+3 tests, 469 / 19 baseline)

Parser: in parse-decl-type, dispatch on the post-= token:
  '|' or Ctor   -> sum type
  '{'           -> record type
  otherwise     -> type alias (skip to boundary)
AST (:type-alias NAME PARAMS) with body discarded. Runtime no-op since
SX has no nominal types.

poly_stack.ml baseline exercises:
  module type ELEMENT = sig type t val show : t -> string end
  module IntElem = struct type t = int let show x = ... end
  module Make (E : ELEMENT) = struct ... use E.show ... end
  module IntStack = Make(IntElem)

Demonstrates the substrate handles signature decls + abstract types +
functor parameter with sig constraint.

lib/ocaml/baseline/poly_stack.ml

@@ -0,0 +1,27 @@
+(* Baseline: polymorphic stack via functor over an Element module *)
+module type ELEMENT = sig type t val show : t -> string end ;;
+
+module IntElem = struct
+  type t = int
+  let show x = Int.to_string x
+end ;;
+
+module Make (E : ELEMENT) = struct
+  let create () = ref []
+  let push x s = s := x :: !s
+  let pop s =
+    match !s with
+    | [] -> None
+    | h :: t -> s := t ; Some h
+  let length s = List.length !s
+  let to_string s =
+    String.concat "," (List.map E.show !s)
+end ;;
+
+module IntStack = Make(IntElem) ;;
+
+let s = IntStack.create () ;;
+IntStack.push 1 s ;;
+IntStack.push 2 s ;;
+IntStack.push 3 s ;;
+String.length (IntStack.to_string s)