Doorways are now placed in different ways, depending on the dimensions
of the walls that they're on. This includes that large walls get two
doorways connecting to the same room.
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.
