Use VariableBaseMSM to optimize decryption

[ggkitsas]

Loop in https://github.com/anoma/ferveo/blob/master/tpke/src/decryption.rs#L67 can be sped up by using arkworks VariableBaseMSM.

The code below shows an initial approach (currently failing with errors):

        use ark_ec::msm::VariableBaseMSM;
        // sum_D_j = { [\sum_j \alpha_{i,j} ] D_i }
        for (D, alpha_j, sum_alpha_D_i) in izip!(shares.iter(), alpha_ij.iter(), sum_D_j.iter_mut())
        {
            let Dj = D.iter().map(|Dij| Dij.decryption_share).collect::<Vec<_>>();
            let alpha_j = alpha_j.iter().map(|a| a.into_repr()).collect::<Vec<_>>();
            *sum_alpha_D_i = VariableBaseMSM::multi_scalar_mul(&Dj, &alpha_j);
        }

[ghost]

It seems there is potential to speed up MSM further, also; see:
https://github.com/Manta-Network/wasm-zkp-challenge/blob/optimized_pippenger/src/pippenger_msm.rs
(above is GPL licensed)