Node constructor is now completely default constructed i.e. no
constructor arguments required. The iterate function takes the queue
by reference, so it can update the caller's state.
Finally, to_string for a Node now uses the node allocator and an index
to print out trees. Seems simpler and more in line with the current
implementation.
Separates the (basically) completed cw_tree implementation in a
separate module so we can spend time implementing graphics in main.
NOTE: nodes are now generated from a node allocator, which just wraps
around the vector. We also take an allocator and queue by reference
in the iterate procedure now.
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.
