Switch to Pippenger MSM

[tarakby]

closes #17
Use Pippenger multi scalar multiplication for E1 points for a faster threshold signature reconstruction (40-50% faster depending on the threshold value).

Example of BenchmarkSignatureReconstruction with a threshold of 40 (on an apple M3 arm64):

Naive MSM:

BenchmarkSignatureReconstruction-8           412           2855833 ns/op

Pippenger MSM:

BenchmarkSignatureReconstruction-8           825           1495744 ns/op

Note: computing E2 public keys from the verification vector in Feldman VSS was not updated to use Pippenger MSM in E2. The current method was compared in timing against Pippenger and turns out to be faster. The current method leverages the polynomial particular case where the degree can be incrementally computed. It also takes advantage of the polynomial input being a small scalar (less than 8 bits).