Collision now uses bits and masks. The main upshot is that ghost and
ice/directional blocks collide much more correctly, now. And turtles
block fireballs.
Also, monsters can now move over "no" signs, and can trample the player
if she's standing on top of an item.
While I was at it, I finished implementing the "bestowal bow", an item
mod (same layer as the "no" sign) that allows any actor to pick up the
item in that tile.
