* gc.c (assign_heap_slot): refactoring variable names.
* gc.c (slot_add_freeobj): added.
* gc.c (heaps_add_freeslot): added.
* gc.c (finalize_list, rb_gc_force_recycle, slot_sweep): use
  `slot_add_freeobj' instead of modifying linked list directly.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41999 b2dd03c8-39d4-4d8f-98ff-823fe69b080e