Add a data structure, based on the zipper comonad, which provides
support for multiple levels, each of which is its own entity map. The
current level is provided by coreturn, which the `entities` lens has
been updated to use. Nothing currently supports going up or down levels
yet - that's coming next.
Add a dungeon level generator, which:
1. generates an infinite sequence of rectangular rooms within the
dimensions of the level
2. removes any duplicates from that sequence
3. Generates a graph from the delaunay triangulation of the centerpoints
of those rooms
4. Generates the minimum-spanning-tree of that delaunay triangulation,
with weights given by line length in points
5. Adds back a subset (default 10-15%) of edges from the delaunay
triangulation to the graph
6. Uses the resulting graph to draw corridors between the rooms, using a
random point on the near edge of each room to pick the points of the
corridors