Simplify per-block attestation processing

[JustinDrake]

Below is a simplified version of process_attestation (the only hard-to-read transaction processing function). It incorporates #698 and #752. It has miscellaneous cleanups and simplifications. It also avoids untriggerable asserts (which goes against the goal of 100% coverage in the executable spec) such as assert get_bitfield_bit(attestation.custody_bitfield, i) == 0b0. I did this mostly for myself but happy to create a PR if people like it.

def process_attestation(state: BeaconState, attestation: Attestation) -> None:
    """
    Process ``Attestation`` transaction.
    Note that this function mutates ``state``.
    """
    assert max(GENESIS_SLOT, state.slot - SLOTS_PER_EPOCH) <= attestation.data.slot
    assert attestation.data.slot <= state.slot - MIN_ATTESTATION_INCLUSION_DELAY

    # Check source epoch and root
    assert (slot_to_epoch(attestation.data.slot), attestation.data.source_epoch) in {
        (get_current_epoch(state), state.current_justified_epoch),   # Case 1: current epoch attestation
        (get_previous_epoch(state), state.previous_justified_epoch)  # Case 2: previous epoch attestation
    }
    assert attestation.data.source_root == get_block_root(state, get_epoch_start_slot(attestation.data.source_epoch))

    # Check crosslink data
    assert attestation.data.crosslink_data_root == ZERO_HASH  # [to be removed in phase 1]
    assert state.latest_crosslinks[attestation.data.shard] in {
        attestation.data.previous_crosslink,  # Case 1: crosslink in state matches previous crosslink
        Crosslink(                            # Case 2: crosslink in state matches current crosslink
            crosslink_data_root=attestation.data.crosslink_data_root,
            epoch=slot_to_epoch(attestation.data.slot)
        )
    }

    # Check custody bits [to be generalised in phase 1]
    assert attestation.custody_bitfield == b'\x00' * len(attestation.custody_bitfield)

    # Check aggregate signature [to be generalised in phase 1]
    participants = get_attestation_participants(state, attestation.data, attestation.aggregation_bitfield)
    assert len(participants) != 0
    assert bls_verify(
        pubkey=bls_aggregate_pubkeys([state.validator_registry[i].pubkey for i in participants]),
        message_hash=hash_tree_root(AttestationDataAndCustodyBit(data=attestation.data, custody_bit=0b0)),
        signature=attestation.aggregate_signature,
        domain=get_domain(state.fork, slot_to_epoch(attestation.data.slot), DOMAIN_ATTESTATION),
    )

    # Cache pending attestation
    pending_attestation = PendingAttestation(
        data=attestation.data,
        aggregation_bitfield=attestation.aggregation_bitfield,
        custody_bitfield=attestation.custody_bitfield,
        inclusion_slot=state.slot
    )
    if slot_to_epoch(attestation.data.slot) == get_current_epoch(state):
        state.current_epoch_attestations.append(pending_attestation)
    else:
        state.previous_epoch_attestations.append(pending_attestation)

[vbuterin]

Definitely looks like an improvement to me!

[cemozerr]

This also seems to fix the issue I just mentioned in #775