BN256 pairing

BN256 pairing library that implements original traits from zkcrypto,

and plus

FieldExt, CurveExt traits that are used in halo2 library.

This implementation is mostly ported from matterlabs/pairing and zkcrypto/bls12-381.

Bench

No assembly

$ cargo test --profile bench test_field -- --nocapture

Assembly (returns rust nightly)

$ cargo test --profile bench test_field --features asm -- --nocapture