* initial attestation aggregation
* fix usage of committee index, vs index in committee; uniformly set trailing/following distance; document how the only-broadcast-if mechanism works better and what aggregation already happens, not otherwise sufficiently clear; use correct BlockSlot across epoch boundaries
* address inconsistent notion of which slot in past to target for aggregate broadcast; follow 0.11.x aggregate broadcast p2p interface topic
* Fix get_slot_signature(...) call after get_domain(...) change required genesis_validators_root
* mark all spec references which aren't dealt with in other PRs as v0.11.1
* update two more spec refs to v0.11.1
* complete (except for get_domain(...)) 0.11.0 beacon chain spec update
* mark compute_start_slot_at_epoch(...), is_active_validator(...), compute_signing_root(...), and get_seed(...) as 0.11.0