First major improvement (and removal) of WorldObjectType.  It's replaced by two classes, Asset and ActorType, and the class hierarchy's been removed.

Tanks are now probably horribly broken, and some copy pasta needs to be fixed.
Tanks were not broken, working fine!  Removed the copy pasta.
Attempted to add post-possession invulnerability.  Doesn't work due to our design cramping up, but it's worth checking in the comments and new constants anyway.