* internal.h (RCLASS_SERIAL): Add RCLASS_SERIAL as a convenience
  accessor for RCLASS_EXT(klass)->class_serial.

* class.c, vm_insnhelper.c, vm_method.c: Use RCLASS_SERIAL

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44098 b2dd03c8-39d4-4d8f-98ff-823fe69b080e