* gc.c (heap_get_freeobj): clear slot->freelist here.
  This means that this slot doesn't have any free objects.
  And store this slot with objspace->heap.using_slot.
* gc.c (gc_before_sweep): restore objspace->freelist
  into objspace->heap.using_slot->freelist.
  This means that using_slot has free objects which are
  pointed from objspace->freelist.
* gc.c (gc_slot_sweep): do not need to clear slot->freelist.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42131 b2dd03c8-39d4-4d8f-98ff-823fe69b080e