The fourth is simply a duplicate of ship_collision for bullets. A
bit redundant, but I figure it's clearer of a definition. Besides,
this opens up the door to possibly making bullets a different struct
type in the future, if that turns out to be desirable.
As a result, you're not stuck with the weakest weapon in the game
anymore. You have to choose between the double rockets, plasma
permanently upgraded in one area, plasma temporarily upgraded in
some areas, or some combination.