Tree: various speedups (#887)

* Tree: various speedups

- make dataclass non-frozen
- use mutate() for cases where a Map is modified multiple times
- remove asserts for cases that would fail immediately anyway

* make frozen depend on __debug__, restore an assert

* Improve depth()

Co-authored-by: Alexandru Fikl <alexfikl@gmail.com>

* opt ancestors

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Co-authored-by: Alexandru Fikl <alexfikl@gmail.com>