Pubsub cleanup

[diondokter]

Exposes the new length functions on the pubs and subs and seals the pubsubbehavior trait (which users shouldn't (need to) touch).

I guess there's more to clean up or at least things I'd have made different now, but there's nothing that really needs more work.

For posterity:

• I think it's weird that DynSubscriber<T> and Subscriber<T> both deref as Sub<Channel, T>.
• Same for publisher
• I'd make the shared functions into a trait and let each of the higher level types implement those traits
• This would be a breaking change, but likely one that doesn't break anyone

[lulf]

LGTM!