Drc permit standard

[steel-judoka]

Fixes #2

Introduces permit-based approvals, which allow a user to approve an allowance via an off-chain signature, rather than using approve() directly. This is inspired by EIP-2612 but is modified to fit Dusk’s crypto and ABI. In particular we don’t use EIP-712, and BLS and the hash function are used for signing and verification respectively.

What was added

• permit(PermitCall) - Sets allowance(owner, spender) = value if the PermitCall contains a valid BLS signature from owner over a deterministic digest of owner, spender, value, nonce, and deadline.
• permit_nonces(owner) -> u64
• domain_separator() -> BlsScalar

Why not EIP-712?

EIP-2612 which introduces permit based approval, uses a signed message with a hash of typed structured data, which is computed via EIP-712. In this implementation we don’t use EIP-712. This is because:

• Dusk uses BLS for signing, not secp256k1, and so the signing scheme and encoding are different.
• We use a single hashing function, which is the hash function exposed through the ABI, abi::hash. This is used off-chain to compute the digest and is also used on-chain. This is simpler and more efficient.