Today, VMATree::register_mapping depends on two worklist arrays which are used for keeping track of the iteration order of the tree. These arrays are re-allocated for each call to register_mapping, which has a measurable performance impact on the tree.

This ticket intends to remedy the situation in two ways:

1. By removing one of the arrays entirely, and instead using merge/split operations for what this array used to be employed for.
2. By saving the remaining array as part of the VMATree instance, reusing it. As the size of this tree grows on an order of log n where n is the number of nodes in the tree, and its elements are small, this is not a large cost.