Enable NestedArray to process nested arrays iteratively instead of recursively.

Problem/Motivation

Taxonomy's TermStorage::loadTree method processes trees using a loop instead of recursion. It would be nice to have a generic form of this solution that could be used for any nested array.

Proposed resolution

The attached patch adds a process() method to the NestedArray class that enables it to run callables on the elements of a nested array without using recursion.