base data for balaur
give balaurs a (temporarily single and random) breath
make balaurs cast all spells simultaneously
store spell types on each head individually
avoid allowing heads to share breath types
balaurs need some resistances

especially poison, since they breathe poison clouds. the others are less
important, although possibly flavorful (are there any monsters with
breath weapons that aren't resistant to the element they breathe?). it'd
be even nicer to have their resistances depend on the heads that are
generated, but i really don't want to have to allocate a ghost_demon
structure just for that.
should set mon->number too, to help reuse some hydra code
give balaurs a hydra-like melee attack
give the balaur's melee attacks a corresponding flavor
handle chopping off balaur heads properly
show the number of heads remaining
make chopping off balaur heads a bit more difficult