Non-animated levels now work

Now we can theoretically play the game on displays where animation
between cells isn't supported.
Remove game class, and move everything in "level" package up a level

It looks like we're not going to use a separate game class to house the
loop, so no point putting level types in a subpackage of roverchip. As a
result cells and sprites all move up a level as well.
Added move key queueing for smoother control

- Movement keys are now queued, so that if a second key is pressed and
then the first is released, the second key will be used for movement.
Add Arduino/LED screen frontend

Outputs a Sokoban game via serial to an LED matrix controlled by an
Arduino.

Still pretty rough, but works!