Adds a default bullet_act proc which calls on_hit

on_hit stores the unique activities of projectiles, which originally only affected mobs, so was only run when a bullet hit a mob, but some projectile weapons like the pulse rifle, gyrojet, and ion rifle use on_hit for the result of the projectile hitting an object, so this would cause projectiles that should explode on hitting things to unceremoniously disappear.  This change allows specific objects to not call on_hit, which makes sense for some some non-solid objects like singularities, but makes objects default to effectively being hit by projectiles.