Experiment: extension 6 of KoalaBear

[TomWambsgans]

An interesting experiment would be to implement and benchmark the extension of degree 6 for KoalaBear with Plonky3 (For instance: degree 5 has been implemented here, but for degree 6 there are more possibilities (tower field)).

Gnark is also working in this direction (see here). @yelhousni likely has valuable insights on how to do this efficiently.

[yelhousni]

Still investigating this but normally Toom3 over Karatsuba mul is the fastest for Fp6 (as Fp2-->Fp6)