classes: use predictable randomness. This allows us to run an op on both
client and server, and if randomness is involved (eg, item drops,
critical hits, losing gear/stats on death) - then it will be equivalent
on both client and server, while still seeming "random". This is
required for the new death mechanic. Kudos to @snicker for providing the
predictableRandom() function. @paglias we can probably stop using _.tmp
now, and simply flag on the client when user drops an item / streak
bonus. @SabreCat this will be used for crits, I'll keep you posted