Use __invoke in std::function internals

Use __invoke in std::function internals

	* include/std/functional (__callable_functor): Remove.
	(_Function_handler::_M_invoke): Use __invoke instead of
	__callable_functor or mem_fn.
	(function::_Callable): Use lvalue in result_of expression.
	(function): Remove TODO comments about allocators.
	* testsuite/20_util/function/cons/refqual.cc: New test.

From-SVN: r239166