The goal was actually only to fix the usages of `val`s and `var`s across the `Scheme`s (cf. #476), but this ended up in being a major overhaul of how the various `Tree`-related classes work together. I found quite a few pieces of code that were, by themselves, neat, but for which much better solutions existed. With these changes, I feel more confident in the stability of the code, and plenty of docs and tests have been added as well. I am aware that #473 will be another overhaul of the `Tree`s, but that won't be until v3.1.x, and I don't want to wait for that. Also, depending on the solution for that issue, the code here might be retained completely anyway.