js-on-sx: js-dict-get-walk falls back to Object.prototype

Object literals didn't carry a __proto__ link, so ({}).toString()
couldn't reach Object.prototype.toString. Added a cond clause: if
the object has no __proto__ AND is not Object.prototype itself,
walk into Object.prototype. Now ({}).toString() works, override
of Object.prototype.toString propagates, and ({a:1}).hasOwnProperty
('a') returns true. built-ins/String: 69/99 → 71/99 (canonical),
71/99 → 74/99 (isolated). conformance.sh: 148/148.

lib/js/runtime.sx

@@ -2596,6 +2596,8 @@
         ((dict-has? obj skey) (get obj skey))
         ((dict-has? obj "__proto__")
           (js-dict-get-walk (get obj "__proto__") skey))
+        ((not (= obj (get Object "prototype")))
+          (js-dict-get-walk (get Object "prototype") skey))
         (else js-undefined)))))
 
 (define