Reduce alt_bn128 precompile gas costs

Status history

1. Draft21 May 2018
2. ⋯Finaldate not observed

Abstract

Changes in 2018 to the underlying library used by the official Go reference implementation led to significant performance gains for the ECADD, ECMUL, and pairing check precompiled contracts on the alt_bn128 elliptic curve. In the Parity client, field operations used by the precompile algorithms were optimized in 2018, and recent changes to the pairing algorithm used by the bn crate have brought…