* Simplify by removing `exit_epoch` in `BeaconState` and `Validator`. (Also less storage for full nodes.)
* Replace with `exit_epoch` when ordering validator with withdrawal. Slight loss of granularity, but that's actually a good thing because it reduces the opportunity for manipulation by proposers.
* (Logic change) Introduce invariant that attestations can be included for `EPOCH_LENGTH`.
* Simplify presentation by related checks into a single line.
* Fix bug whereby `intersection` (replaced by `slashable_indices `) could be non-empty, but no validator gets penalized because they were all already penalized.
* Rename `SlashableVote` to `SlashableAttestation` (for consistency with `Attestation`, and to avoid having "Vote" be a synonym of "Attestation").
* Rename `slot_included` to `inclusion_slot` for consistency with the `_slot` (and `_epoch`) postfix convention
* Homogenise ordering of fields in `Attestation` and `SlashableAttestation` objects (one has an `aggregation_bitfield` instead of the other's `validator_indices`).
* Homogenise ordering of fields in `Attestation` and `PendingAttestation` objects (one has an `aggregate_signature` instead of the other's `inclusion_slot`).
* Light presentation cleanup
* Removed comment beside `latest_block_roots` "Needed to process attestations, older to newer" because it is no longer accurate
* Clarified per-slot processing
* Casper slashing -> attester slashing
Adds a minimum slot number large enough that integer underflows involving epochs and slots will not happen; simplifies some logic that was more complex to handle them.
