Sync with pasta_curves 0.5.0 about halo2curves

[CPerezz]

While porting secp256k1 I noticed the following:
Now PrimeField requires to be passed MULTIPLICATIVE_GENERATOR and ROOT_OF_UNITY.

MulGen is easy to derive using in Sage:

sage: GF(0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f).primitive_element()
3

From this, we should be able to derive the Root of Unity. By doing:

Zp=Integers(0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f)
sage: mul_gen = Zp(3)
sage: root_unity = Zp((mul_gen^(modulus -1)))
sage: root_unity
1

This means, that INV_ROOT_UNITY which was equal to 0 previously in the implementation was wrong as it should be 1.

Currently investigating this.