As well as simpifying the code by removing duplication, this means that
we only need to touch std::__invoke_r if we need to implement changes to
INVOKE<R>, such as those in P0932R0.

	* include/bits/std_function.h (_Simple_type_wrapper): Remove.
	(_Function_handler): Remove partial specializations for void return
	types and pointers to member.
	(_Function_handler::_M_manager): Adapt to removal of
	_Simple_type_wrapper.
	(_Function_handler::_M_invoke): Use __invoke_r instead of __invoke.
	* include/std/functional (_Bind_result::__enable_if_void)
	(_Bind_result::__disable_if_void): Remove sfinae helpers.
	(_Bind_result::__call): Use __invoke_r and remove overloads for void
	return types.
	* include/std/future (__future_base::_Task_state::_M_run)
	(__future_base::_Task_state::_M_run_delayed): Use __invoke_r and
	change return type of lambda expressions.

From-SVN: r271174