The left, currently processed (centre) and right fractions are
returned by iterate. This allows us to see exactly what fractions
have been generated/worked on in every iteration.
Abstracting the interface more, such that callers can use functions
rather than accessing internals directly, allows me to refactor the
allocator without having to do a ton of edits all across the source
tree.
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.
