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/sum_squares.ml

@@ -0,0 +1,6 @@
+(* Baseline: imperative loop summing squares *)
+let total = ref 0 ;;
+for i = 1 to 10 do
+  total := !total + i * i
+done ;;
+!total