[shell] Add state_document convenience function
[ent] Let Biped fire bullets (currently without weapon :|)
[ent] Fix a bullet bug (there are more)
[ent] Fix another bullet bug
[ent] Prevent infinite loop in bullet casting
[ent] Bullet logging
[ent] Destroy bullets when not ricocheting
[ent] Prevent backwards bullet collisions.  Bullets seem to be more or less working now.
[ent world] Implement shooting sound (currently empty)
[ent world] Add gun item
[shell vis] Gun item seems to behaviorally work.  Also make models not crash if a seg isn't posed
[ent world] Draw gun on model
[world] Add shoot SE