Make MAX_POLY_DEGREE into a const generic

[teor2345]

We want to have polynomials of different maximum sizes, and different moduli based on those sizes.

• [x] We can’t multiply polynomials with different maximum sizes, because we don’t know what modulus to choose
• [x] When we split in karatsuba, the maximum sizes should be the same, because the modulus is the same (done in #32)