get_shuffling and get_attesters_and_proposers

[jannikluhn]

From the spec:

def get_attesters_and_proposer(crystallized_state, active_state, skip_count):
    attestation_count = min(len(crystallized_state.active_validators), ATTESTER_COUNT)
    indices = get_shuffling(active_state.randao, len(crystallized_state.active_validators))
    return indices[:attestation_count], indices[attestation_count + skip_count]

This fails if attestation_count + skip_count >= len(active_validators) which is possible either if skip_count is large or if len(active_validators) < ATTESTER_COUNT. I think one should be able to fix this by using (attestation_count + skip_count) % len(active_validators) as the proposer index (but it would mean that the proposer is also an attester, not sure if that's an issue).

The implementation in this repository is a little bit more complicated because of the sample parameter, resulting in an infinity loop in get_shuffling: At some point remaining will be zero so the inner loop breaks, but the outer loop will still continue. Proposed fix here is to just get rid of the sample complexity and just yield each o[i] as soon as it's replaced.

Finally, this check seems to be useless: https://github.com/ethereum/beacon_chain/blob/master/beacon_chain/state/state_transition.py#L59.

[jannikluhn]

Same as #18