rationals, etc with reference ids.
* ext/psych/lib/psych/visitors/yaml_tree.rb: ditto
* ext/psych/lib/psych/visitors/to_ruby.rb: loading complex numbers,
  rationals, etc with reference ids.
* test/psych/test_object_references.rb: corresponding tests

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@33679 b2dd03c8-39d4-4d8f-98ff-823fe69b080e