Optimize pairing product computation by moving exponentiations to G1.

[dfaranha]

What the title says. :) This version passes all tests and is approximately 2x faster in the benchmarks.
A shorter but slightly slower alternative can be found below. This is joint work with @aniagut.

// ProdPair calculates the product of pairings, i.e., \Prod_i pair(ni*Pi,Qi).
func ProdPair(P []*G1, Q []*G2, n []*Scalar) *Gt {
        if len(P) != len(Q) || len(P) != len(n) {
                panic("mismatch length of inputs")
        }

        mi := new(ff.Fp12)
        out := new(ff.Fp12)
        out.SetOne()

        affineP := affinize(P)
        for i := range affineP {
                affineP[i].ScalarMult(n[i], &affineP[i]);
                affineP[i].toAffine();
                miller(mi, &affineP[i], Q[i])
                out.Mul(out, mi)
        }

        e := &Gt{}
        finalExp(e, out)
        return e
}

[armfazh]

Thanks @aniagut and @dfaranha