* include/std/functional: (_Function_base::_Function_base()): Use
	nullptr instead of literal zero.
	(function::operator=(nullptr_t)): Likewise.
	(_Function_handler::_M_invoke): Use perfect forwarding for _ArgTypes.
	(function::_Invoker_type): Likewise.
	* testsuite/20_util/function/invoke/forwarding.cc: New.

From-SVN: r216849