[HyperNova] Use Sparse Polynomial & MLE reprs

[CPerezz]

We're currently using sparse representations for our matrixes for CCS.
It would be nice to actually be able to save space by also using sparse representations for MLE of matrixes & polynomials as well as for the polynomials themseleves.

On that way, since usually a lot of the evaluations or coefficients of these elements are 0, we can save up a lot of space.