js-new-call Object had set obj.__proto__ correctly, but then the
__callable__ returned a fresh (dict), which js-new-call's "use
returned dict over obj" rule honoured — losing the proto. Added
is-new check (this.__proto__ === Object.prototype) and return
this instead of a new dict when invoked as a constructor with
no/null args. Now new Object().__proto__ === Object.prototype.
built-ins/Object: 37/50 → 41/50. conformance.sh: 148/148.