* Bring forward changes to withdrawability from phase 1
* The `WITHDRAWABLE` flag is removed; instead, a validator's withdrawability is determined through the `withdrawable_epoch` field (renamed and re-purposed from `withdrawal_epoch` which was not used)
* When a validator passes through the withdrawal queue, the `prepare_validator_for_withdrawal` function does not let them withdraw immediately; instead, they have to wait `MIN_VALIDATOR_WITHDRAWAL_EPOCHS`. This extra minimum delay serves no value in phase 0, but is crucial for phase 1 as the period between a validator passing through the queue and the validator being eligible to withdraw is where proof of custody challenges can come in; adding it in phase 0 is only half a line of code so easier to add it now.
* If a validator is penalized, they are no longer subject to the exit queue; instead, their `withdrawable_epoch` is set `LATEST_PENALIZED_EXIT_LENGTH` into the future and this is used to determine when the validator can withdraw
* Changes the eligibility condition for a transfer to use the `withdrawable_epoch`
The reason to do this is that it makes it calculable from inside an attestation how many epochs the attestation spans over, which is needed for proof of custody reasons. It's a relatively small change and so arguably easier to do now than to do as a patch in phase 1.
Note that this changes the meaning of latest_crosslink.epoch, from the epoch when the latest crosslink was included to the epoch that the latest crosslink was for. This affects the line:
* `state.latest_crosslinks[shard].epoch > state.validator_registry_update_epoch` for every shard number `shard` in `[(state.current_epoch_start_shard + i) % SHARD_COUNT for i in range(get_current_epoch_committee_count(state))]` (that is, for every shard in the current committees)
But this may actually make it _more_ correct, as it means that in the case where >512 shards are processed per epoch, and so a committee from the previous epoch could get finalized in the current epoch, that would no longer count toward every shard having received a "new" crosslink.
