Please support creating a Receiver from a Sender and vice versa

Receiver and Sender both contain the same Shared structure. I'd love to have a `Sender::make_receiver` function that produces a Receiver from a Sender (and vice versa with `Receiver::make_sender`), to allow storing only one in a data structure rather than storing the pair. For precedent, see https://docs.rs/tokio/latest/tokio/sync/watch/struct.Sender.html#method.subscribe .