Idea: AST tree without changing representation

[CAD97]

It recently hit me that Pest's QueueableToken list is roughly isomorphic to the flat tree that Rowan uses.

That means we can, with no change to the internal representation, provide a typed tree API similar to Rowan's.

[dragostis]

This actually really interesting and something I've though of a while ago. I think it's certainly worth a shot.