added queuing and dequeuing operations
flood fill is now graph based
Instead of iteratively searching for available nodes, keep track of the nodes with adjacent vertices that haven't been searched yet.  This should yield much better performance and enable floodfill to solve even if the full maze hasn't been solved.
removed ff_recompute_target
I don't think there is a way to actually recompute the target value without researching the graph.  This is becuase this BFS produces a tree which can't be easily restructured to have the new node as the root
made bracket usage consistent