Check attestation shuffling when producing blocks

[michaelsproul]

Issue Addressed

Closes #845

Proposed Changes

During block production, use the fork choice block DAG to check that the implied shuffling for an attestation is the same as the state on which the block is being produced. This ensures that the signature check that was done upon inserting into the op pool remains valid, and prevents attestations from forks with different shufflings from appearing on chain with seemingly invalid signatures.

Additional Info

To avoid having the op pool contain a reference to fork choice, the get_attestations function takes a filtering closure. I thought this was quite neat (and functional 😏)!

TODO

• Tests for different fork lengths
• Unit tests for attestation_shuffling_is_compatible
• Consider combining the checks for attestations sharing a beacon block root. There could be up to 128 attestations, but likely only a handful of unique roots that need checking.

[paulhauner]

I didn't go through the tests in detail but the main logic looks good to me!

Squerge at will.