Stop passing size around everywhere.

* Declares a maze-size var into core.
* Removes size as an arg to most fns.
* Sets the maze-size var via a defonce when we generate a maze.

Also, I had a realization: if we created walls around the perimeter of
the maze, we wouldn't need to track size (nor do any bounds checking at
all!). I created #4 to record this idea.