ocaml: phase 6 Hashtbl.iter / Hashtbl.fold (+2 tests, 494 total)

New host primitive _hashtbl_to_list returns the entries as a list of
OCaml tuples — ('tuple' k v) form, matching the AST representation
that the pattern-match VM (:ptuple) expects. Without that exact
shape, '(k, v) :: rest' patterns fail to match.

Hashtbl.iter / Hashtbl.fold in runtime walk that list with the user
fn. This closes a long-standing gap: previously Hashtbl was opaque
once values were written (we could only find_opt one key at a time).

  let t = Hashtbl.create 4 in
  Hashtbl.add t "a" 1; Hashtbl.add t "b" 2; Hashtbl.add t "c" 3;
  Hashtbl.fold (fun _ v acc -> acc + v) t 0   = 6

lib/ocaml/eval.sx

@@ -115,7 +115,23 @@
       (list "_hashtbl_mem"
         (fn (t) (fn (k) (has-key? (nth t 0) (str k)))))
       (list "_hashtbl_length"
-        (fn (t) (len (keys (nth t 0))))))))
+        (fn (t) (len (keys (nth t 0)))))
+      ;; _hashtbl_to_list: returns [(k, v); ...] as a list of pairs.
+      ;; Keys are returned as the stringified form used internally.
+      (list "_hashtbl_to_list"
+        (fn (t)
+          (let ((d (nth t 0)) (out (list)))
+            (begin
+              (define ks (keys d))
+              (define loop
+                (fn (xs)
+                  (when (not (= xs (list)))
+                    (begin
+                      (append! out
+                        (list "tuple" (first xs) (get d (first xs))))
+                      (loop (rest xs))))))
+              (loop ks)
+              out)))))))
 
 (define ocaml-env-lookup
   (fn (env name)