Adds a new type of turf, which is a cracked asteroid floor. It is functionally a space tile, which means air can escape through the cracks and into the void. These cracks can be plugged by building a floor over them using rods and tiles. The cracks are generated randomly along with the asteroid whenever a floor is placed, with a 1 in 10 chance of it being a cracked floor.
Created a global list to track base turfs for explosions/shuttle moves.
Remaps the asteroid to be a moonlet. Tidies up some references to 'asteroid', removes moonbase from the accessible z level list.
