feat(data/polynomial/derivative): if `p` is a polynomial, then `p.derivative.nat_degree ≤ p.nat_degree - 1` (#12948)
feat(data/polynomial/derivative): if `p` is a polynomial, then `p.derivative.nat_degree ≤ p.nat_degree - 1` (#12948)

I also golfed the proof that `p.derivative.nat_degree ≤ p.nat_degree`.





leanprover-community/mathlib,leanprover-community/mathlib