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
this should allow to build timelines and check access permissions
quickly. my only concern is keeping thousands, if not even millions, of
rows always with full strings. future optimizations may look into an
intermediary table to map ids to integers and optimize this table's
storage
