deprecate make-vtable-vtable

deprecate make-vtable-vtable

* libguile/struct.h:
* libguile/struct.c (scm_make_vtable_vtable): Deprecate, as you can
  handle most of the use cases with make-vtable, and we don't want to
  promote the creation of new roots to the type hierarchy.
  (scm_i_make_vtable_vtable): The internal replacement.