Cleanups
* Use `state.slot` over `current_slot`.
* Specify `new_status`
* Segregate ejection processing
Bug fixes:
* Put ejections at the end of the per-slot processing (TODO: per-slot processing should come *after* per-epoch processing)
* Immediately exit ejections
* Include `ACTIVE_PENDING_EXIT` as an active state
* Renamed two `State` to `BeaconState`
Initial pass for the migration from a per-block state transition function to a per-slot state transition function. More simplifications and cleanups can be made.
This change is to avoid deposits from fully consuming the allowable balance churn, preventing exits from being processed. And vice versa with deposits/exits swapped.
A bunch of cleanups and bug fixes were made along the way.
`next_start_shard` does not need to be part of `state` (it's already available via `(state.shard_committees_at_slots[-1][-1].shard + 1) % SHARD_COUNT`)
Changelog (nothing major):
* Clean up constants
* Add to table of contents
* Rename a few
* Split into more categories (e.g. "Deposit contract" and "Initial values")
* Replaced `SQRT_E_DROP_TIME ` by `INACTIVITY_PENALTY_QUOTIENT`
* Put all the data structures in "Data structures", including specials
* Move deposit contract after "Data structures"
* `ShardAndCommittee` => `ShardCommittee` (and related changes)
* `aggregate_sig` => `aggregate_signature`
* `justified_slot_bitfield` => `justification_bitfield`
* Use double-ticks everywhere in Python docstrings
* Clean up deposit contract a bit
* Bug fix: ensure uniqueness of the `ChainStart` log
