Implement inverse of unit polynomials modulo prime power

[user202729]

Just one easy-to-implement case here.

Flint has functions https://flintlib.org/doc/fmpz_mod_poly.html#c.fmpz_mod_poly_invmod https://flintlib.org/doc/nmod_poly.html#c.nmod_poly_invmod but that assumes the modulus is prime, which is not the case here.

Note that the result is unique because if f ⋅ g = 1 and f ⋅ g' = 1 then f ⋅ (g-g') = 0, so g ⋅ f ⋅ (g-g') = 0, but g ⋅ f = 1, so g=g'.

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[user202729]

Maybe in the future we can implement something like https://math.stackexchange.com/questions/300170/ideals-of-mathbbzx or https://dl.acm.org/doi/10.1145/800205.806353 instead, which would be fully general (note that (ℤ/nℤ)[x] = ℤ[x] / (n)`