* ShufflingRef approach to next-epoch validator duty calculation/prediction
* refactor action_tracker.updateActions to take ShufflingRef + beacon_proposers; refactor maybeUpdateActionTrackerNextEpoch to be separate and reused function; add actual fallback logic
* document one possible set of conditions
* check epoch participation flags and inactivity scores to ensure no penalties and MAX_EFFECTIVE_BALANCE to ensure rewards don't matter
* correctly (un)shuffle each proposer index
* remove debugging assertion