[WIP] Consolidate stat bonus setter and modifiers.

Added:
enum - STAT { STR, DEX, PER, INT }
function - void Creature::set_stat_bonus(STAT stat, int amt)
function - void Creature::mod_stat_bonus(STAT stat, int amt)

Didn't realize on the last PR that my fork was outdated. Back up to HEAD and ready to go :)