Gens 1 and 2: Proper stat reduction calculation
This is huge since there's a change in -1 Snorlax's speed that is relevant to Gen 2.

See:
http://www.smogon.com/forums/threads/stat-reduction-bug.3520407/
https://github.com/iimarckus/pokered/blob/master/engine/battle/core.asm#L7856
https://github.com/kanzure/pokecrystal/blob/master/battle/effect_commands.asm#L6608