Making zeroization optional

[brycx]

As the discussion in #59 also points out, zeroizing values allocated on the stack doesn't really give us any guarantees that copies aren't left around. Right now, zeroization is enabled by default and so even if users don't consider this a part of their threat-model, they still pay the performance penalty.

Perhaps it's more sensible to make zeroization of data optional, still keeping it on by default but allowing opt-out if users specifically don't care about this.