defining a smob or port type no longer mucks exports of (oop goops)

defining a smob or port type no longer mucks exports of (oop goops)

* libguile/goops.c (DEFVAR): Remove this helper macro, replacing its
  uses with scm_module_define, but without scm_module_export.
  (create_basic_classes, scm_init_goops_builtins): Update callers.
  (make_class_from_template, make_class_from_symbol): Change to not
  define variables for classes.  This affects ports, struct classes, and
  smob classes.

* module/oop/goops.scm: Explicitly list our exports, so there is no more
  trickery happening in C.
  (find-subclass): Private helper to grub the class hierarchy, so we can
  define bindings for smobs, ports, etc.  Use to define the classes that
  goops.c used to define -- probably a subset, but it's better to have
  them listed.