Clarify death effects and grouping

[mciul]

Continuing from #397, let's refactor death effects for Kerkerkruip 11.

• Separate "serving" from "accompanying." If a monster serves someone, their "master" claims kills for them. But "serving" does not affect soul absorption.
• Go over killing/death effects and look for redundancy or needless complexity. I have a really hard time following these effects and I suspect they could be made simpler.
• Some group behaviors might be better off as special cases rather than generalized effects. Every group behaves differently, and that's what's cool about them.
• Add a new group of monsters. I have some ideas but they haven't crystallized yet. I'll create a new issue for that when I've thought about it some more.