goops.c no longer knows about <class> slot allocation

goops.c no longer knows about <class> slot allocation

* libguile/goops.c (scm_class_of): Access "redefined" slot by name in
  the case where we need to change the class of an instance.
  (scm_sys_goops_early_init): Move up capture of class-precedence-list
  so SCM_SUBCLASSP can use it.

* libguile/goops.h (SCM_CLASS_CLASS_LAYOUT, scm_si_redefined)
  (scm_si_direct_supers, scm_si_direct_slots, scm_si_direct_subclasses)
  (scm_si_direct_methods, scm_si_cpl scm_si_slots)
  (scm_si_getters_n_setters, SCM_N_CLASS_SLOTS, SCM_OBJ_CLASS_REDEF):
  Remove.  Now C code has no special knowledge about the layout of
  GOOPS classes.
  (SCM_SUBCLASSP): Use scm_class_precedence_list to get CPL.
  (SCM_INST, SCM_ACCESSORS_OF): Remove unused macros that were
  undocumented and nonsensical.