feat(data/polynomial/zcoeff): define zcoeff + API

[adomani]

This PR will likely be subsumed by defining and using Laurent polynomials: the coefficients of a Laurent polynomial are naturally indexed by an integer.

ℤ-coefficients of polynomials

Let R be a (semi)ring, let f : R[X] be a polynomial and let n : ℤ be an integer.

This file defines f.zcoeff n : R, the coefficient of a polynomial in degree n : ℤ,
where we assign coefficient 0 to every negative degree. Thus, p.zcoeff n coincides with
p.coeff n, when 0 ≤ n and equals 0 otherwise.

Zulip

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