Manipulate GOOPS vtable flags from Scheme, for speed

Manipulate GOOPS vtable flags from Scheme, for speed

* libguile/goops.h: Remove unimplemented declarations of
  scm_make_next_method, scm_sys_invalidate_method_cache_x, and
  stklos_version.
  (scm_sys_invalidate_class_x): Remove helper definition.  This was
  exported in the past but shouldn't have been.

* libguile/goops.c (scm_sys_make_vtable_vtable): Rename from
  scm_sys_make_root_class, and don't do anything about flags.
  (scm_sys_bless_applicable_struct_vtables_x, scm_class_p)
  (scm_sys_invalidate_class_x): Remove; we do these in Scheme now.
  (scm_init_goops_builtins): Define Scheme values for vtable flags.

* module/oop/goops.scm (vtable-flag-goops-metaclass)
  (class-add-flags!, class-clear-flags!, class-has-flags?)
  (class?, instance?): New definitions.
  (<class>): Add GOOPS metaclass flags from Scheme.
  (<applicable-struct-class>, <applicable-struct-with-setter-class>):
  Add flags from Scheme.
  (make, initialize): Add class flags as appropriate.
  (class-redefinition): Clear the "valid" flag on the old class.
  (check-slot-args): Use instance? instead of a CPL check.