Add a highlight grid to Map, with locations set to True if that point should be
drawn with a "highlight" treatment.
Add the highlight graphic_dtype to all Tiles.
- Generate creates rooms and corridors, and apply applies them to a tile grid.
- Add up and down stairs generation to the Room Generators.
- Clean up Room.wall_points and Room.floor_points to make it easier to
write a generic apply() method on RoomGenerator
- Move room generators to map.generators.room
- Move corridor generators to map.generators.corridor
Generators have a generate() method that generates the things they place,
and an apply() method that applies their objects to a grid of tiles.
