* gc.c (scm_init_storage): init `scm_gc_registered_roots'.

* gc.c (scm_init_storage): init `scm_gc_registered_roots'.
(scm_igc): mark from them, too (precisely, not conservatively!).

* root.h (scm_gc_registered_roots): new object in
scm_sys_protects.

* hooks.c (scm_create_hook): call `scm_gc_protect_object' instead
`scm_protect_object'.  shouldn't call it at all, though, it seems.

* gc.c (scm_[un]protect_object): deprecated.
(scm_gc_[un]protect_object): new names for scm_[un]protect_object.
(scm_gc_[un]register_root[s]): new.

* gc.h: add prototypes for scm_gc_[un]protect_object,
scm_gc_[un]register_root[s].