coly_lisp_object now dows not create new nil and t values anymore

Fixed an error where a nil value could not be used as the default
value as a keyword argument since coly_lisp_object would be invoked
before assigning nil as the default argument, producing a separate nil
value, which then fails the nil tests becase it lives in a different
location as Memory::nil.

bin/tests/evaluation_of_default_args.slime

@@ -0,0 +1,7 @@
+((lambda (:keys k1 :defaults-to (+ 1 2 3))
+   (assert (= k1 6))))
+
+((lambda (:keys k1 :defaults-to ())
+   (when k1
+     (assert ()))
+   (assert (= k1 ()))))