Better arrow collision detection.
Implement heads-up-display with health bar

The hud is tracked in a special variable on the play state, even though it
technically ducktypes as an actor. This is so the hud is always drawn on
top of the other actors.

Resolve #7
Start work on inventory.
Fix get_bulk() bug
Draw debug touching circles around actors.
Items cannot be collided.
Add rock item for testing.
Switch arrow to using util.touching