SLOT-EXISTS-P is supposed to work (in the sense of not signalling an error)
on any objects, not just standard objects. In addition, it is supposed
to report if the object has slots. Since structures and conditions have
slots, and are objects, it should report correctly on these.
I've added some tests to gcl/ansi-tests/slot-exists-p.lsp for this.
Oddly, the tests on structures fail, but succeed if you reload the file
(thereby redefining the structures). The condition test forms just