merge revision(s) 39601 and 39602(partially): [Backport #7983]

	* class.c (rb_prepend_module): check redefinition of built-in optimized
	  methods.  [ruby-dev:47124] [Bug #7983]

	* vm.c (rb_vm_check_redefinition_by_prepend): ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39912 b2dd03c8-39d4-4d8f-98ff-823fe69b080e