feat(data/polynomial/mirror): Central coefficient of `p * p.mirror` (#14096)
feat(data/polynomial/mirror): Central coefficient of `p * p.mirror` (#14096)

This PR adds a lemma `(p * p.mirror).coeff (p.nat_degree + p.nat_trailing_degree) = p.sum (λ n, (^ 2))`.
I also rearranged the file by assumptions on the ring `R`.





leanprover-community/mathlib,leanprover-community/mathlib