feat: Hash to Curve chip for BLS12-381 (G2)

[nulltea]

Add support for hashing to curve inside circuit. Based on draft-irtf-cfrg-hash-to-curve-16.

Relies on axiom-crypto/halo2curves#8. Should be merged after #175 and #176.

Scope

• HashInstructions trait to abstract hash function chip implementation used for message expansion.
• ExpandMessageChip trait to allow multiple messages expansion algorithms.
• ExpandMsgXmd provided implementation of ExpandMessageChip based on section 5.3 of draft-irtf-cfrg-hash-to-curve-16.
• HashCurveExt trait that extends CurveExt with constants specific to hash to curve operations. Implemented for bls12_381::G2.
• HashToCurveInstructions trait extends EccChip with methods required by HashToCurveChip. Implemented for EccChip<'chip, F, Fp2Chip<'chip, F>> (G2).
• HashToCurveChip implements hashing to curve functionality for any EccChip variant that implements HashToCurveInstructions.

This implementation allows support hashing for new curves easily. Only HashCurveExt and HashToCurveInstructions traits must be additionally implemented.

Benchmarks

k=19 lookup_bits=18:

Gate Chip | Phase 0: 742340 advice cells
Total 143 fixed cells
Total range check advice cells to lookup per phase: [105180, 0, 0]