Wall Limit to Antistack change

This is another step in retooling wall spells into far more active and
interesting spells now that the 'zone in' problem, that had been acting
as the primary restraint on wall power, has been dealt with.

This commit changes the summoning limit in a room to 200, up from 18.
Instead, rooms now have a new anti-stacking function which prevents more
than one element of the same type to be placed on the same spot. This
should prove to be a far more organic limit on wall spell use, along
with vigor cost, mana cost, and duration rebalancing.

The general intent from this point will be to lower casting times on many wall spells to enable their use midcombat and give them more useful shapes or deployments, but also shorten their max durations to between 30 and 60 seconds. Rather than boring stationary placement in doors (which no longer works), these spells should be active and powerful - but temporary - area-control mechanisms. There's no reason for them to last 2-4 minutes anymore.*

* There will certainly be a longer duration wall spell reserved for holding ghosts as long as ghosts are a thing. That's still a very important use.

* This is also a subtle boost to Lightning Wall and Faren masters. With antistack, you cannot stack two firewalls anymore - but you can stack a firewall and a lightning wall.