Change how status effects are applied, and fix dead monsters leveling up

Status effects are pre-calculated chance before deciding
which to apply, meaning that they will happen on a less
often but more solid basis, and criticals will now boost
the chance of existing status effects by 250%.

Leveling-up monsters need to perform their death check
just before leveling, so that they don't die and then
level up after the delay runs out on their corpse.