feat(nifs): `compute_G`

[cyphersnake]

fn compute_G(
    beta: F,
    delta: F,
    alpha: F,
    ws: &[PlonkWitness],
    S: PlonkStructure // S.gates
) -> UnivariatePoly<F> {
  // 1. calculate L_0(X)*ws[0]+L_1(X)*ws[1]+L_2(X)*ws[1]+...
  // over dk+1 points of the cyclic group
  // 2. pow_i()*f_i()
  // 3. ifft
   todo!()
}