define-object-amendment does not check for reserved words override

[reiniervandijk]

Macro define-object-amendment does not check for override of reserved words. Hence, a user can naively break gendl completely by doing something like.

(define-object foo ())

And then:

(define-object-amendment foo ()
  :computed-slots
  ((parent nil)))

[genworks]

Merged with #158.