This PR is spun out of #427 and focuses entirely on moving the API structs to a "shared state" pattern, reducing the prevalence of Arc in the API without any functional changes.

This one is very subjective so I don't mind discarding it, but I felt that the prevalence of Arc in the API was adding noise that is seldom relevant to the average user. In this PR, I've refactored the API a little so that Node, Subscription, Publisher, Client, and Service each have a respective _State structure which is public and represents the underlying instance. At the same time, Node, Subscription, etc are now type aliases of the form

• pub type Node = Arc<NodeState>;
• pub type Subscription<T> = Arc<SubscriptionState<T>>;

This does not lead to any actual change in the structure or memory management of any of these types; it's a purely superficial renaming. But it reduces how often Arc is tossed around in the API and thereby streamlines the 95% (my own estimate) of use cases where Arc is just an implementation detail. At the same time, it does not interfere with the remaining 5% of use cases where the Arc is relevant, e.g. if a user wants to hold onto a Weak<Node> for some reason.