Resistance system rewrite

Resistances were a bit wonky and historically prone to permanent errors
that left players invincible or weak. Rather than rely on adding and
removing permanent resistances as gear is equipped or unequipped,
resistances are now calculated anew at every resistance check through:

- Battler base resistance
- Battler object attributes
- Battler equipment
- Battler equipment object attributes
- Battler personal enchantments

Radius enchantments still modify resistance 'on the fly' during an
attack as they will always have range, safety, and attack type
conditionals. They won't show up in the players' stat window or any base
current resistance calculations for this reason.

Area enchantments do not currently modify resistances at all.

And yes, this new resistance system checks for gear and object atts on
gear worn by monsters, such as troops. It's entirely possible to outfit
a Necromancer troop, for example, in gear as strong as that worn by
players - and even have them drop it once killed.