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/expected.json

@@ -13,6 +13,7 @@
   "module_use.ml": 3,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
+  "poly_stack.ml": 5,
   "queens.ml": 2,
   "quicksort.ml": 44,
   "sum_squares.ml": 385,