vm.c: hide singleton class of frozen-core

* vm.c (Init_VM): hide also the singleton class of frozen-core, not
  only frozen-core itself.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43232 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: use str_duplicate

* string.c (rb_str_resurrect): use str_duplicate(), which does
  completely same.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e