Removed some unused vars.
Removed level.player.vars.unlocked_items.
This necessitated the removal of three global functions related to vehicles and powerups, all of which were unused.
It also caused the removal of spawn_item, which was used by two objects. The mushroom_hopper_blue merely had the function removed, as the object crashes when thrown and thus quite visibly needs work. The milgram_slab, however, used the function to spawn an item when it died. The slab had no prototypes, so I changed it's prototype to hittable and gave it just enough code to call the function to spawn items when it dies. The poor slab could really use some love…
Changed damage type function to return string or null.