ocaml: phase 6 Hashtbl (+6 tests, 332 total)

Backing store is a one-element list cell holding a SX dict; keys
coerced to strings via str so int/string keys work uniformly. API:
create, add, replace, find, find_opt, mem, length.

_hashtbl_create / _hashtbl_add / _hashtbl_replace / _hashtbl_find_opt /
_hashtbl_mem / _hashtbl_length primitives wired in eval.sx; OCaml-side
Hashtbl module wraps them in lib/ocaml/runtime.sx.

lib/ocaml/eval.sx

@@ -66,7 +66,30 @@
       ;; Print: prints to host stdout via println.
       (list "print_string"  (fn (s) (begin (print s) nil)))
       (list "print_endline" (fn (s) (begin (println s) nil)))
-      (list "print_int"     (fn (i) (begin (print (str i)) nil))))))
+      (list "print_int"     (fn (i) (begin (print (str i)) nil)))
+      ;; Hashtbl primitives (one-element list cell holding a dict).
+      ;; Keys are coerced to strings via `str` so any value type works
+      ;; as a key (matches Hashtbl's polymorphic-key semantics).
+      (list "_hashtbl_create" (fn (n) (list {})))
+      (list "_hashtbl_add"
+        (fn (t) (fn (k) (fn (v)
+          (begin
+            (set-nth! t 0 (merge (nth t 0) (dict (str k) v)))
+            nil)))))
+      (list "_hashtbl_replace"
+        (fn (t) (fn (k) (fn (v)
+          (begin
+            (set-nth! t 0 (merge (nth t 0) (dict (str k) v)))
+            nil)))))
+      (list "_hashtbl_find_opt"
+        (fn (t) (fn (k)
+          (cond
+            ((has-key? (nth t 0) (str k)) (list "Some" (get (nth t 0) (str k))))
+            (else (list "None"))))))
+      (list "_hashtbl_mem"
+        (fn (t) (fn (k) (has-key? (nth t 0) (str k)))))
+      (list "_hashtbl_length"
+        (fn (t) (len (keys (nth t 0))))))))
 
 (define ocaml-env-lookup
   (fn (env name)