merges r20747 and r20802 from trunk into ruby_1_9_1.

* test/ruby/test_metaclass.rb: new test case for metaclass hierarchy.

* class.c (make_metametaclass): new function. extracted from
  rb_make_metaclass.

* class.c (rb_make_metaclass): uses make_metametaclass when called for a
  metaclass.

* class.c (rb_singleton_class): creates a meta^(n+2)-class in
  addition to a meta^(n+1)-class when called for a meta^(n)-class.
  This is because the returned meta^(n+1) class must acts as an instance of
  Class, metaclass of Class, ..., meta^(n+1)-class of Class,
  Module, metaclass of Module, ..., meta^(n+1)-class of Module,
  Object, metaclass of Object, ..., meta^(n+2)-class of Object,
  BasicObject, metaclass of BasicObject, ..., meta^(n+2)-class of
  and BasicObject even when Class, Module, Object or BasicObject has
  not have its meta^(i)-class yet.

git-svn-id: http://svn.ruby-lang.org/repos/ruby/branches/ruby_1_9_1@20814 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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