BLS signature v4

[mratsim]

This upgrades the repo to support the full BLS v04 signatures:

Specs

• Diff v03 and v04: https://tools.ietf.org/rfcdiff?url1=https://tools.ietf.org/id/draft-irtf-cfrg-bls-signature-03.txt&url2=https://tools.ietf.org/id/draft-irtf-cfrg-bls-signature-04.txt
• Ethereum specs: Bump IETF BLS spec version draft 03 -> draft 04 ethereum/consensus-specs#2080

Public API changes

• privToPub and secretToPublic out of place proc have been changed to publicFromSecret so that a boolean can be returned and catch invalid secret keys == 0.
• The aggregate API now returns a boolean to indicate aggregating an empty openarray[Signature] instead of asserting.
  This addresses: [SEC] Infinity Public Key and Signature #76
  Note that we used to have 3 aggregate overloads for the following cases:
  • combine a valid signature with another valid signature
  • combine a valid signature with an array of valid signatures
  • combine an array of valid signatures.
    [SEC] Infinity Public Key and Signature #76 revealed that it wasn't clear that in the first 2 cases
    the aggregation precondition of having a non-empty aggregation was always true.
    This is now mentioned as a comment and the last overload has also been renamed aggregateAll and now has an in-place API + boolean.

Notable changes

A zero secret-key or public key is checked at deserialization (see supranational/blst#11) for discussion.
This slightly differ from KeyValidate in the spec which includes zero-key and subgroup check. (related #42)
The spec does mention that KeyValidate MAY be cached (see #90)