all subrs are gsubrs

all subrs are gsubrs

* libguile/gsubr.c (create_gsubr, create_gsubr_with_generic): Always
  create gsubrs -- never the specialized tc7 types. Allow gsubrs to have
  generics, there doesn't seem to be any reason not to.

* libguile/macros.c (scm_make_synt):
* libguile/values.c (scm_init_values):
* libguile/eval.c (scm_init_eval):
* libguile/gc.c (scm_init_gc): Use scm_c_define_gsubr instead of
  scm_c_define_subr.

* libguile/goops.c (scm_class_of): Allow gsubrs to be primitive
  generics.