* gc.c: Removed #include "libguile/guardians.h".

* gc.c: Removed #include "libguile/guardians.h".
(scm_before_gc_c_hook, scm_before_mark_c_hook,
scm_before_sweep_c_hook, scm_after_sweep_c_hook,
scm_after_gc_c_hook): New C level hooks.
(scm_after_gc_hook): New Scheme level hook.
(scm_gc_sweep): Moved scanning of weak vectors to weaks.c.
(scm_igc): Moved initialization of scm_weak_vectors and the call
to scm_guardian_gc_init to respective module.
(scm_mark_weak_vector_spines): Moved to weaks.c;
Call to scm_guardian_zombify moved to guardians.c;
Run scm_before_gc_c_hook, scm_before_sweep_c_hook,
scm_after_gc_c_hook at appropriate places.
(scm_init_gc): Initialize scm_after_gc_hook.