Monster-character unification

This gets rid of `Creature::hit` and uses `Creature::deal_damage` instead. This does not change any mechanic, it's a simple replacement of the call to `Creature::hit` with the content of that function.

Additionally (and that is a changed), I changed some damage types to the more appropriate ones (heat from fire, acid from acid, ...), monsters consider the acid proof flag on when being hurt through `deal_damage` - this makes the check in fields.cpp redundant.