Add reverse traversal

[lgfa29]

This PR implements reverse tree traversal.

It introduces a new type of Iterator called ReverseIterator that can move through a tree in reverse order using the Previous() and SeekReverseLowerBound() methods.

It also adds a new WalkBackwards() method to Node that works the same way as Walk() but in reverse, going from the largest to the lowest values.

[banks]

Looking really good!

I suggest a fuzz test to exercise more of the edges and possibly found a bug but I could be wrong!

[lgfa29]

@banks I added fuzz tests for the ReverseIterator. It was super helpful, so thanks for the tip.

I noticed that the recurse bug is only triggered when the seek starts from a non-root node, so I added two tests. There's a comment explaining why this is the case. I hope the explanation is clear enough, otherwise I can expand it. It's kind of an unusual case, but I guess not impossible, so worth testing.

The two tests are very similar, but they are different enough that I couldn't find a nice way to re-use things, I hope that's OK.

[banks]

Great job Luiz!

I think it could be possible to work the two fuzz tests into one with some random bool input that chooses wether or not to try to start at a non-root node. Then both paths would be exercised with high probability in any given run (check runs 1000 iterations by default IIRC).

That said, I don't think it's especially problematic to have them separate either so accepting for now.