remove requirements on _pairing_ and `ark-poly-commit`

changelog

• remove ark-poly-commit from the dependencies
• remove the old setup.rs
• add temporary foo.rs which define (some details are ommited for brevity)
  • struct Powers<F, G>
  • struct Commitment<F, G>
  • fn build_powers<F, G>(...) -> Powers<F, G>
  • fn commit<F, G, P>(powers: &Powers<F, G>, polynomial: &P) -> Commitment<F, G>
  • tests
• foo.rs is then renamed to setup.rs
• two new KomodoError variants have been created to replace ark_poly_commit::Error
  • DegreeIsZero
  • TooFewPowersInTrustedSetup(usize, usize)
• finally, all the past mentions to E: Pairing and setup::random have been replaced with F, G and setup::build_powers respectively