Added kitten movement, tweaked cannon

Added a move-kitten and next-world function to move the kitten according
to it's speed. The kitten is currently unstoppable.
Added "(on-tick next-world)" clause to big-bang.
Changed dimensions of cannon's arm to prevent it from protruding out of
the back of the cannon base.
Made cannon arm slightly wider and adjusted starting (x, y); rotation
functionality preserved (good job Charles).