channel helper for subscription

[hecrj]

This PR implements a channel helper function to create a Subscription.

The channel function signature looks like this:

pub fn channel<I, Fut, Message>(
    id: I,
    size: usize,
    f: impl Fn(mpsc::Sender<Message>) -> Fut + MaybeSend + Sync + 'static,
) -> Subscription<Message>
where
    I: Hash + 'static,
    Fut: Future<Output = Never> + MaybeSend + 'static,
    Message: 'static + MaybeSend,

The helper creates an mpsc channel with the given size and provides the Sender end to the closure f. The resulting Future will be run concurrently and the Subscription will produce the messages published to the Sender.

Notice how the Future produces a Never type. This effectively means the Future can never finish by itself. It can only be terminated by the Application.

This is a simpler version of unfold where the future can simply run indefinitely with a loop while publishing to the Sender instead of continuously returning to produce messages.

The unfold helper has also been changed to always produce a Message when returning, as opposed to allowing None to only change state. If state changes are necessary without producing output, the channel helper is recommended.

Closes #1553.