class.c: no fstring singleton class

	* class.c (singleton_class_of): prohibit fstrings from creating
	  singleton classes.
	  temporary measure for [ruby-dev:49867] [Bug #12923]
	test_fstring.rb: fix exception

	* test/-ext-/string/test_fstring.rb (test_singleton_class): fix
	  expected exception class.  [ruby-dev:49867] [Bug #12923]
	vm_eval.c: fstring instance_eval

	* vm_eval.c (singleton_class_for_eval): enable fstring singleton
	  class for instance_eval.  [ruby-core:78116] [Bug #12930]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@58157 b2dd03c8-39d4-4d8f-98ff-823fe69b080e