ocaml: phase 6 stdlib slice (List/Option/Result, +23 tests, 248 total)

lib/ocaml/runtime.sx defines the stdlib in OCaml syntax (not SX): every
function exercises the parser, evaluator, match engine, and module
machinery built in earlier phases. Loaded once via ocaml-load-stdlib!,
cached in ocaml-stdlib-env, layered under user code via ocaml-base-env.

List: length, rev, rev_append, map, filter, fold_left/right, append,
iter, mem, for_all, exists, hd, tl, nth.
Option: map, bind, value, get, is_none, is_some.
Result: map, bind, is_ok, is_error.

Substrate validation: this stdlib is a nontrivial OCaml program — its
mere existence proves the substrate works.

lib/ocaml/eval.sx

@@ -588,16 +588,28 @@
         (loop decls)
         result))))
 
-;; ocaml-run — convenience wrapper: parse + eval.
+;; ocaml-run — convenience wrapper: parse + eval. Layers the stdlib env
+;; (List, Option, Result) underneath the empty env so user code can use
+;; `List.map` etc. without explicit setup.
+;; Variable guarded so eval.sx is loadable without runtime.sx. runtime.sx
+;; sets ocaml-stdlib-env once loaded; before that, fall back to the empty
+;; env so the existing tests continue to work without stdlib.
+(define ocaml-stdlib-env nil)
+(define ocaml-base-env
+  (fn ()
+    (cond
+      ((not (= ocaml-stdlib-env nil)) ocaml-stdlib-env)
+      (else (ocaml-empty-env)))))
+
 (define ocaml-run
   (fn (src)
-    (ocaml-eval (ocaml-parse src) (ocaml-empty-env))))
+    (ocaml-eval (ocaml-parse src) (ocaml-base-env))))
 
 ;; ocaml-run-program — evaluate a program (sequence of decls + bare exprs).
 ;; Threads an env through decls; returns the value of the last form.
 (define ocaml-run-program
   (fn (src)
-    (let ((prog (ocaml-parse-program src)) (env (ocaml-empty-env)) (last nil))
+    (let ((prog (ocaml-parse-program src)) (env (ocaml-base-env)) (last nil))
       (begin
         (define run-decl
           (fn (decl)