[Ready] Hash to curve draft 7 + BLS signature draft 2

[mratsim]

This updates our library to Eth v0.12.x.

A toggle BLS_ETH2_SPEC=="v0.11.x" or BLS_ETH2_SPEC=="v0.12.x" is provided for backward compatibility with the witti testnet (on 0.11.3) and fuzzing

The test vectors are:

• ETH2 test vectors
• Test vectors from https://tools.ietf.org/html/draft-irtf-cfrg-bls-signature-07
• Test vectors added in draft revision in the past 4 days:
  • https://github.com/cfrg/draft-irtf-cfrg-hash-to-curve/pull/255
  • https://github.com/cfrg/draft-irtf-cfrg-hash-to-curve/pull/266

Notable:

• ETH2 Test format change for signature aggregation
• Empty signature and zero signature discussion that leads to consensus split: https://github.com/ethereum/eth2.0-specs/issues/1713
• The domain separation tag is now a static string to avoid allocations and involving the GC in crypto. This has the side benefits of easing threading.

[mratsim]

Before merging, we have 2 possibilities:

Default to 0.12.x or default to 0.11.x

[tersec]

Before merging, we have 2 possibilities:

Default to 0.12.x or default to 0.11.x

It seems like 0.11.x is the status quo and would enable immediate drop-in, since 0.12 won't be ready otherwise for some days.