* include/ruby/ruby.h: make Symbol objects frozen.
  [Feature #8906]
  I want to freeze this good day, too.
* test/ruby/test_eval.rb: catch up this change.
* test/ruby/test_symbol.rb: add a test to check frozen symbols.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42974 b2dd03c8-39d4-4d8f-98ff-823fe69b080e