* Finished implementing link planning for mazes. Doors aren't placed yet
because that's up to Decorators and those haven't been implemented yet.
* Added bounding walls to mazes.
* Added decay effects to mazes.
Rewrote portions of our maze generation code to use RoomData. This
provides an object that unifies all room data instead of having it
spread across various data structures and linked loosely by hash maps.
We'll need this to implement the remaining generation features.
Added code to minimize the number of doorways that involve dropping
through the floor. Added a DisjointSet class as part of the
implementation. I also split the maze construction process into two
classes (MazeDesigner and MazeBuilder) to make it clearer.
