Duplicated mul_by_nonresidue function in Bn256 field extensions

[davidnevadoc]

The extensions of Bn256 base field Fq2 and Fq6 are constructed choosing a quadratic/cubic non-residue in their respective base field. Each of these extensions implement the function multiply_by_nonresidue, which multiplies an element by a non-residue element of the extension field.

However, these extensions each have another function that is identical:

• Fq2 has mul_by_xi
  

  halo2curves/src/bn256/fq2.rs

  Line 300 in 6e2ff38

  pub fn mul_by_xi(&mut self) {

• Fq6 has mul_by_v.
  

  halo2curves/src/bn256/fq6.rs

  Line 268 in 6e2ff38

  pub fn mul_by_v(&mut self) {

The remaining extension, Fq12, does not seem to have such duplicated function.
These functions are not used anywhere, I think they are just duplicates and should be removed for clarity.

Edit:
Moreover, the following comments should be corrected:

halo2curves/src/bn256/fq6.rs

Line 258 in 6e2ff38

/// Multiply by cubic nonresidue v.

This v is the non-residue used for the quadratic extension [Fq12 : Fq6]. It is a quadritic, not cubic, non-residue.
The same applies for:

halo2curves/src/bn256/fq2.rs

Line 277 in 6e2ff38

/// Multiply this element by quadratic nonresidue 9 + u.

This should be cubic, not quadriatic.

[davidnevadoc]

Solved in #99