SCM_GENERIC_METHOD_CACHE macro splits from SCM_ENTITY_PROCEDURE

SCM_GENERIC_METHOD_CACHE macro splits from SCM_ENTITY_PROCEDURE

* libguile/goops.h (SCM_GENERIC_METHOD_CACHE)
  (SCM_SET_GENERIC_METHOD_CACHE): Two new macros; the same as
  SCM_[SET_]ENTITY_PROCEDURE, but more reflecting the reality of the
  generic hack.

* libguile/eval.i.c:
* libguile/goops.c:
* libguile/objects.c:
* libguile/vm-i-system.c: Use the new macros when it is appropriate to
  do so.