Add dungeon level generation

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

shell.nix

@@ -18,11 +18,7 @@ let
       overrides = (self: super: {
         ghc = super.ghc // { withPackages = super.ghc.withHoogle; };
         ghcWithPackages = self.ghc.withPackages;
-        # eww https://github.com/NixOS/nixpkgs/issues/16394
-        generic-arbitrary = pkgs.haskell.lib.appendPatch
-          super.generic-arbitrary
-          [ ./build/generic-arbitrary-export-garbitrary.patch ];
-      });
+      } // (import ./haskell-overlay.nix { inherit nixpkgs; }) self super);
     }
     else packageSet
   );