ocaml: phase 4 'let open M in body' local opens (+3 tests, 478 total)

Parser detects 'let open' as a separate let-form, parses M as a path
(Ctor(.Ctor)*) directly via inline AST construction (no source slicing
since cur-pos is only available in ocaml-parse-program), and emits
(:let-open PATH BODY).

Eval resolves the path to a module dict and merges its bindings into
the env for body evaluation. Now:

  let open List in map (fun x -> x * 2) [1;2;3]   = [2;4;6]
  let open Option in map (fun x -> x + 1) (Some 5) = Some 6

lib/ocaml/eval.sx

@@ -697,6 +697,14 @@
                                 (fill (rest xs)))))))))
                   (fill bindings)
                   (ocaml-eval body env2))))))
+        ((= tag "let-open")
+          ;; `let open M in body` — extend env with M's bindings, eval body.
+          (let ((path-expr (nth ast 1)) (body (nth ast 2)))
+            (let ((mod-val (ocaml-resolve-module-path path-expr env)))
+              (cond
+                ((dict? mod-val)
+                  (ocaml-eval body (ocaml-env-merge-dict env mod-val)))
+                (else (error (str "ocaml-eval: let open on non-module: " mod-val)))))))
         ((= tag "let-rec")
           ;; Tie the knot via a mutable cell when rhs is function-typed.
           ;; The placeholder closure dereferences the cell on each call.