Beginnings of <slot> slot definition class

Beginnings of <slot> slot definition class

* module/oop/goops.scm (define-macro-folder): Factor out this helper.
  (fold-class-slots): Implement using define-macro-folder.
  (fold-slot-slots): New definition, for slots of <slot-definition>.
  (define-slot-indexer): New helper.  Use to define indexes for slots of
  <class> and of <slot>.