Integrate Milagro impl of BLS signature aggregation

[mkalinin]

Current implementation

There is an existing trivial signature aggregation algorithm that is used as a stub for signature aggregation and verification by Beacon chain processing and attesting parts.

To Do

Replace that naive implementation with BLS12-381 algorithm implemented in Milagro library. Measure a time that is required to aggregate and verify signatures with usage of that integration.

References

• https://github.com/zkcrypto/pairing/tree/master/src/bls12_381/
• BLS12-381 spec https://github.com/ethereum/eth2.0-specs/blob/master/specs/bls_verify.md
• benchmarks by @bkolad https://docs.google.com/spreadsheets/d/1Z55ra7TKw-K5-Qa-bWqD4YLfccamnSuLpGNhAhp7jRI/edit#gid=1834751607
• benchmarks by Nimbus team https://github.com/status-im/nim-beacon-chain/issues/8#issuecomment-436957467