* gc.c: introduce RZombie to manage zombie objects.
  Rewrite finalizing logics with this type.
* gc.c (gc_verify_internal_consistency): verify zombie (finalizing)
  objects count.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46348 b2dd03c8-39d4-4d8f-98ff-823fe69b080e