Expand liches' magical repertoire.

They get spells in a fashion reminiscent of panlords, with spells
from two schools and an escape spell.

The exact spell sets could use adjustment; thus the branch this is
landing on initially.
pick some random spells from the list instead of all of them
simplify
simplify some more
ensure liches always get a reasonable number of spells

also, don't disallow choosing the same spell set twice - nothing wrong
with that now that we don't just pick a fixed number of schools
add a bunch more spell options
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
try to encourage more consistently-themed spell sets
normalize lich casting frequency
have normalize_spell_freq maintain frequency weighting
adjust some spell frequencies

primary spells should be cast most often, and frequencies in general
should be randomized a bit
these two spells work now
remove a few uninteresting spells
only try to place ignite poison with other poison spells