halo2ecc-s

Re-implement an ecc circuits with halo2.

Optimizations

1. Generate witness outside of synthesize.
2. Support multi-threading for witness generation.
3. Use 3 limbs for bn256 Fp over Fr integer.
4. Reduce lookup.
5. Use select chip to reduce bisect points in MSM.

Pairing

1. Support bn256 pairing check over bn256 Fr circuit. See test_bn256_pairing_chip_over_bn256_fr for usage.
2. Support bls12_381 pairing check over bn256 Fr circuit. See test_bls12_381_pairing_chip_over_bn256_fr for usage.