Check base field

[wborgeaud]

• Check that wires are in the base field when working over an extension field.
• Open Zs polynomials at zeta and g*zeta.
• Rewrite of the ListPolynomialCommitment API to be more PLONK specific.

[wborgeaud]

It's a bit annoying having to remove these tests, but since the FRI API is now very PLONK specific it doesn't make sense to update these tests since they'll end up being the same as those in commitment.rs.
If we need FRI for something other that polynomial commitments in the future, we could add some kind of FriInitialData trait and make the FRI API more general.

[dlubarov]

I feel this is getting pretty complicated; I tried to refactor a bit here, though with limited success. I guess there's only so much we can do.

• I tried using the powers() iterator to supply all the powers of alpha, instead of tracking cur_alpha and poly_count. Since (currently) we multiply a few things by the same power of alpha, I had to clone the iterator in a few places.
• Added an unsalted_evals helper
• Converted subgroup_x once at the beginning
• Added a comment

What do you think? Feel free to pick any changes you like and ignore the rest.

[wborgeaud]

Thanks for the tweaks, they're all great! I've merged them all.