[WIP] move attestation index outside signed message

[dapplion]

Motivation

ZK circuits attempting to proof Casper-FFG on Ethereum today would benefit greatly from having to verify less signatures.

On an optimal case with 100% agreeing participation, in a network with at least 262144 active validators, one needs to verify 32*64 * 2/3 ~ 1365 attestations to reach a 2/3 threshold.

Ethereum validators cast two votes at once: LMD GHOST vote and Casper-FFG vote. In the perfect example case above, during a single epoch all Casper-FFG votes are identical, and LMD GHOST votes differ per slot. So the ideal minimum number of vote combinations to verify to reach a 2/3 threshold is 32 * 2/3 ~ 22.

The issue arises from validators actually signing 3 things:

1. LMD GHOST vote (beacon_block_root, slot). Note I include slot in case we move to (block, slot) voting
2. FFG vote (source, target)
3. Committee index (index)

class AttestationData(Container):
    slot: Slot
    index: CommitteeIndex
    # LMD GHOST vote
    beacon_block_root: Root
    # FFG vote
    source: Checkpoint
    target: Checkpoint

This 3rd signed item, the committee index, causes equal votes to produce different signing roots.