no reason to actually use spell type enums here anymore
improve lich spell selection

this moves the split entirely to usefulness rather than school. i'll add
back in some checks to encourage similar-schooled spells later.
simplify
give regular liches a primary spell too

and give ancient liches a chance of a second one
make banishment also an emergency spell option

it was in the existing lich spell sets, so
randomize the number of spells that liches get
actually, banishment is not all that interesting as a primary spell

it's fine as an emergency spell, but magic-resistable things aren't very
interesting late-game, generally