Add curve BLS12-381. Targets ff = "0.13.0".

Sources:

• https://github.com/privacy-scaling-explorations/bls12_381 (all types, majority of trait impls, and tests)
• https://github.com/privacy-scaling-explorations/halo2curves/blob/support_bls12-381/src/bls12_381/mod.rs (endomorphism)

The decision to copy implementation from bls12_381 crate is explained by the numerous additional trait impls needed to be compatible with halo2-lib. See bellow.

Additional changes

• implement ff::PrimeField for Fp, Scalar, Fp2.
• implement misc traits for halo2lib compatibility for Fp, Scalar, Fp2
• implement ff::Field for Fp2 including sqrt_ratio.
• implement CurveAffine and CurveAffineExt for G1 and G2
• switch to LE encoding for Fp, G1Affine and G2Affine for compatibility with halo2lib
  • BE encoding is kept in a separate methods to remain compatible with test vectors, and since BE is more commonly used for BLS12 (e.g. Ethereum)
• use local arithmetic.rs funcs and a derive module instead of https://github.com/privacy-scaling-explorations/bls12_381/blob/main/src/util.rs