Switch to classes for Dictionary, List and Item

[evert]

The tuple syntax is clunky and we can significantly improve the developer experience by introducing a few classes such as Dictionary, List and Item.

It might be nice to keep the original bare parsers around because they implement the test suite and RFC parsing/serializing structure exactly. So hopefully we can add this in a way that doesn't break backwards compatibility, but otherwise this might be a 3.0 version