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/runtime.sx

@@ -530,6 +530,24 @@
        | Some v -> v
      let mem t k = _hashtbl_mem t k
      let length t = _hashtbl_length t
+
+     (* iter / fold over (k, v) pairs. Keys come back as their string
+        representation since the host coerces all keys via `str`. *)
+     let iter f t =
+       let rec go xs =
+         match xs with
+         | [] -> ()
+         | (k, v) :: rest -> f k v; go rest
+       in
+       go (_hashtbl_to_list t)
+
+     let fold f t acc =
+       let rec go xs a =
+         match xs with
+         | [] -> a
+         | (k, v) :: rest -> go rest (f k v a)
+       in
+       go (_hashtbl_to_list t) acc
    end ;;
 
    module Map = struct