* gc.c: change the counting method for young objects.
  clear counter at the beggining of every GC and
  count promoted (infant->young) objects.
  Some promotions (infant->young) are transition of promoting to old
  objects. We should not count such promotions.
  With this technique, we don't need to check young objects
  at obj_free().



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46336 b2dd03c8-39d4-4d8f-98ff-823fe69b080e