* gc.c: slim a path of newobj_of().
* gc.c (objspace): add a new field objspace::freelist, which contains
  available RVALUEs.
* gc.c (newobj_of): simply call new function `get_freeobj()'.
  get_freeobj() returns objspace::freelist. If  objspace::freelist
  is not available, refill objspace::freelist with a slot pointed by
  objspace::heap::free_slots.
* gc.c (before_gc_sweep): clear objspace::freelist.
* gc.c (slot_sweep): clear slot::freelist.
* gc.c (heaps_prepare_freeslot): renamed to heaps_prepare_freeslot.
* gc.c (unlink_free_heap_slot): remove unused function.
* gc.c (rb_free_const_table): remove unused function.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42005 b2dd03c8-39d4-4d8f-98ff-823fe69b080e