ocaml: phase 5.1 baseline OCaml programs (5/5 pass) + lookahead boundary

lib/ocaml/baseline/{factorial,list_ops,option_match,module_use,sum_squares}.ml
exercised through ocaml-run-program (file-read F). lib/ocaml/baseline/
run.sh runs them and compares against expected.json — all 5 pass.

To make module_use.ml (with nested let-in) parse, parser's
skip-let-rhs-boundary! now uses has-matching-in? lookahead: a let at
depth 0 in a let-decl rhs opens a nested block IFF a matching in
exists before any decl-keyword. Without that in, the let is a new
top-level decl (preserves test 274 'let x = 1 let y = 2').

This is the first piece of Phase 5.1 'vendor a slice of OCaml
testsuite' — handcrafted fixtures for now, real testsuite TBD.

lib/ocaml/baseline/option_match.ml

@@ -0,0 +1,8 @@
+(* Baseline: option type + pattern matching *)
+let safe_div a b =
+  if b = 0 then None else Some (a / b) ;;
+let result =
+  match safe_div 20 4 with
+  | None -> 0
+  | Some x -> x ;;
+result