<class> accessors implemented in Scheme

<class> accessors implemented in Scheme

* libguile/goops.c (scm_class_p): New internal helper, exported to
  goops.scm.
  (scm_class_name, scm_class_direct_supers, scm_class_direct_slots):
  (scm_class_direct_subclasses, scm_class_direct_methods):
  (scm_class_precedence_list, scm_class_slots): Dispatch to Scheme.
  (scm_sys_goops_early_init): Capture <class> accessors.

* module/oop/goops.scm (define-class-accessor): New helper.
  (class-name, class-direct-supers, class-direct-slots):
  (class-direct-subclasses, class-direct-methods)
  (class-precedence-list, class-slots): Define in Scheme.
  (compute-std-cpl, compute-cpl): Move lower.