Eip2333 updates + drop BLS_ETH2_SPEC toggle + drop Hash-to-curve v5

[mratsim]

This updates nim-blscurve with the following changes:

• Better in line with BLS signature draft v2: https://tools.ietf.org/rfcdiff?url1=https://tools.ietf.org/id/draft-irtf-cfrg-bls-signature-00.txt&url2=https://tools.ietf.org/id/draft-irtf-cfrg-bls-signature-02.txt

  • privToPub deprecated in favor of secretKeyToPubkey (spec skToPk). Converting secret key to public key was just an unspecified helper before.
  • CoreAggregateVerify now uses KeyValidate/publicKey subgroup checks

• EIP2333 https://eips.ethereum.org/EIPS/eip-2333 and keyGen in BLS signature draft 2

  • see:
  • https://github.com/ethereum/EIPs/pull/2748,
  • https://github.com/ethereum/EIPs/pull/2744
  • https://github.com/ethereum/EIPs/pull/2743
  • HKDF_mod_r has an additional key_info parameter and the the key derivation i slightly different.

The changes only affect new key generation, existing keys are not impacted.