no more "impl ..." hell, each trait has associated types so that we know
it's a "Self::Link" or a "Self::Actor", but in practice they can both be
a "serde_json::Value" and thus we can change its type. also Node::Array
is now a Vec<T> rather than Vec<Node<T>> because it makes more sense.
Node is Iterable and will yield zero (Empty|Link), one (Object) or many
(Array) Ts
