Add carcasses on death events

[aaronjewell]

Resolves #103

Add a death event that triggers on starvation and death by health loss. If the dying entity has a new carcass component, spawn an entity of the creature type held in that component. Remove nutrtition to fullness transfer on attack death, and instead decrease nutrition of the attacked by the attack damage of the attacker. This allows for entities that do not spawn carcasses (i.e. plants) to still provide nutrition to their attacker, and prevents live creatures (i.e. herbivores) that do spawn carcasses from providing any nutrition while alive.

[erlend-sh]

Thanks! Evoli doesn’t have an active maintainer at the moment, so even though this can’t be reviewed closely I’ll just merge it so it doesn’t go stale.