WIP Move all polynomials operations in one place

The goal is to help solve https://gitlab.com/nomadic-labs/privacy-team/-/issues/129 and improve the code quality of the code base. To achieve it we move all operations touching polynomials in a folder polynomial containing two file coefficients.ml, our former polynomial library and evaluations.ml, which consider polynomials as an evalution on the subgroup of roots of unity.