* Added `activation_slot`, `exit_slot`, `penalized_slot`, `withdrawal_slot`, use these to determine if a validator is active
* Universal min activation/exit delay of 256 slots
* Min exit time of ~1 day, but penalization delays this to ~18 days
* Penalty calculation period of `[time penalized - 18 days, time penalized + 18 days]`; made the total penalties array fixed size and wraparound to make calculation more fine-grained
* Processes withdrawals in all epochs, not just dynasty-changing epochs
* Change `get_shuffling` function to take slot as argument

Not yet done:

* Removed `shard_committees` from the state
* Removed persistent committees from the state

Co-Authored-By: vbuterin <v@buterin.com>

Cleanups

* (typo) Remove `get_new_validator_registry_delta_chain_tip` from table of contents
* (typo) Update "Routines for updating validator status" in table of contents
* Update `FAR_FUTURE_SLOT` from `2**63` to `2**64 - 1`
* Put more constants in "Initial values", homogenise
* Cleanup note formatting
* Remove `ZERO_BALANCE_VALIDATOR_TTL` logic (to be possibly reintroduced in phase 2).
* Cleanup `min_empty_validator_index`
* Rename `deposit` to `amount` in `process_deposit` and `DepositData`.
* (typo) Remove new line under `process_penalties_and_exits`
* (typo) "Status codes" => "Status flags" in the table of contents
* (typo) `(state.slot - EPOCH_LENGTH) % LATEST_RANDAO_MIXES_LENGTH` => Use `SEED_LOOKAHEAD` instead.
* Put `state.validator_registry_latest_change_slot = state.slot` in `update_validator_registry`.
* Use `GENESIS_SLOT` for `last_poc_change_slot=0` and `second_last_poc_change_slot=0`.

Bugfixes

* (typo) `validator_exit` => `exit.validator_index`
* Separate initial deposits and initial activations to avoid double activations
* Replace `proposer.status != EXITED_WITH_PENALTY` with `validator.penalized_slot > state.slot` in two different places.
* Replace `status == EXITED_WITH_PENALTY` with `validator.penalized_slot <= state.slot` (and validator active) in two different places.

Also trim trailing whitespace