Nilpotence

Add a DecidableNilpotent class.

I'm working on a fork of `polynomial` that uses `algebra` instead of `vector-space`. Having this class allows me to create a handy default definition for `DecidableUnit (Poly a)`: a polynomial over a commutative ring is a unit iff its constant term is a unit and all of the coefficients of its non-constant terms are nilpotent.