Size half, fusion rifle, disengage

Changes:

• Added fusion rifle to Kobold
• Goblin and Kobold are size half. This doesn't really significantly alter gameplay, but technically they can now be walked over by enemies and don't cause engagement to larger things
• Implemented Disengage action 
• Kobold slag spray now blocks movement and provides hard cover — block off entrances to your hearts content

Bugfixes:

• Fixed softlock when you rammed a slag pile. It was technically moving, but was too slow to see.
• Fixed knockback not interrupting movement
• Fixed no enemy waves spawning