Move <class> initialization to Scheme

Move <class> initialization to Scheme

* libguile/goops.c (scm_sys_make_root_class): Just make the
  vtable-vtable, and leave initialization to Scheme.

* libguile/struct.c (scm_i_make_vtable_vtable): Change to take a full
  list of fields, not just the extra fields.
  (scm_init_struct): Adapt to scm_i_make_vtable_vtable change.

* module/oop/goops.scm (<class>): Compute layout for <class>, and
  initialize <class> from here.