CPU performance issues in diff iterator

Hookalyzer's development is stalled because of severe performance issues in the diffing iterator.

The current iterator implementation is highly inefficient. It walks data trees (trees/branches being either an object or an array) top-down. From each branch value in the tree, it fully descends to each reachable leaf. This results in a some kind of polynomial-ish worst-case running time, primarily determined by the height of the tree. (I think - it might not actually be this bad, but even if not it's somewhere around quadratic for the worst-case)

A better implementation could work from the bottom of the trees up and never redescend. In theory, we could get it up to O(n), worst-case running time.

Because Drupal 8 passes data structures with >50 levels of nesting through alters (cough queries cough views cough), this hits us very, very hard.