* reorder ssz
* split into hash_trees and ssz_serialization, roughly, for hashing and
IO
* move bitseqs into ssz (from stew)
* clean up imports
* docs, imports
* switch state transition caching usage to shuffled active validator indices to match EpochRef
* refactor the EpochRef -> StateCache transformation; elide pointless mapIt
* limit state passed between get_beacon_committee(...) and compute_committee(...)
* tweaks
* plumbing between block pool and state transition functions around active validator indices and committees
* have shared epochrefs followed by blockref tree while allowing for skipped slots
* factor out the epoch info extraction; document how the EpochRef follows forks
* in makeBeaconBlock - use rollback instead
* in tests - this helps state_sim give more accurate data and makes it
30% faster
* fix some usages of raw BeaconState
Mamy Ratsimbazafy
Dustin Brody
Jacek Sieka