Tweak freezing

[nitaku]

• [ ] Shooting a frozen ship should kill it
• [ ] Momentum should not be conserved for frozen ships
• [x] Rocks should convert into ice if touched by a freeze ray
• [x] Ice rocks should freeze other rocks
• [ ] Ships should not be thrown outside the battlefield
• [ ] Frozen ships should be static (no fancy angles and whatnot)
• [ ] Frozen ships should be destroyed if touched (e.g. Race and Freezing Death)
• [ ] No freeze bombs (not good in e.g., Freezing Death)