Add two more screensaver tests
Make OverloadedStrings a default extension
Use the debug option to exit the game session after a given time
Test animations in testDefense
Define multi-floor lifts, similar to starcases
Replace Descend k by Ascend (-k)
Let levels go up, not down

There's artificial gravity on this toroid spaceship and the player
starts in the outermost level.
Lower human IQ until pathfinding is implemented