Derives the ability to use this struct of Senders and AsyncSenders to
call .send or .send_async directly as if using one of the included
Senders or AsyncSenders.
Derives the ability to convert an object into this struct of Sender and
AsyncSenders, as long as the object can be converted into each individual
Sender or AsyncSender.
The conversion is done by calling .as_multi_sender() or .into_multi_sender().