Add isPos() and isNeg() functions. Add Z.lift - same as Z.of, but doesn't clone Zs.
Use Z.lift() across all the static functions. Add a few missing static variants.
Use Z.lift() in a few places where I'm not altering the passed-in value, and so don't need to clone. Add more static variants.
Pull constructor's final return up into the only clause that hits it.