slot-ref, slot-set! et al bypass "using-class" variants

slot-ref, slot-set! et al bypass "using-class" variants

* module/oop/goops.scm (slot-ref, slot-set!, slot-bound?, slot-exists?):
  Bypass slot-ref-using-class, slot-set-using-class!, and so on.  Those
  interfaces are public and have to check that the class is indeed a
  class, they should check that the object is an instance of the class,
  and so on, whereas if we get the class via class-of we know that the
  invariant holds.