Abstract away the tree

[keks]

The query code only cares about having a tree that has something like RangedNodes. Currently these are generated on the fly, but I'd like to experiment with embedding the ranges directly in the tree. I'd also like to experiment with using persistent backends for the tree, so making the tree abstract would be really nice.

• figure out what behaviour the query code needs
• make a trait that exposes that behaviour
• adapt the query code to use that trait
• implement the trait with the current code
• proptest that the behaviour is the same