An index is a pointer, and they don't change if the vector decides to
reallocate internally unlike the bastardised pointers I was rolling up
before. This simplifies design a bit.
Pops an item off the queue and generate left and right children for it,
if those are empty. Then push those children into the queue for the
next iteration.
NOTE: Because we're using a queue, this does a breadth first
generation of the tree, which is what we want.
