Altair consensus changes and refactors (#2279)

## Proposed Changes

Implement the consensus changes necessary for the upcoming Altair hard fork.

## Additional Info

This is quite a heavy refactor, with pivotal types like the `BeaconState` and `BeaconBlock` changing from structs to enums. This ripples through the whole codebase with field accesses changing to methods, e.g. `state.slot` => `state.slot()`.


Co-authored-by: realbigsean <seananderson33@gmail.com>

consensus/state_processing/src/per_epoch_processing/base.rs

@@ -0,0 +1,76 @@
+use super::{process_registry_updates, process_slashings, EpochProcessingSummary, Error};
+pub use crate::per_epoch_processing::validator_statuses::{
+    TotalBalances, ValidatorStatus, ValidatorStatuses,
+};
+use crate::per_epoch_processing::{
+    effective_balance_updates::process_effective_balance_updates,
+    historical_roots_update::process_historical_roots_update,
+    resets::{process_eth1_data_reset, process_randao_mixes_reset, process_slashings_reset},
+};
+pub use justification_and_finalization::process_justification_and_finalization;
+pub use participation_record_updates::process_participation_record_updates;
+pub use rewards_and_penalties::process_rewards_and_penalties;
+use types::{BeaconState, ChainSpec, EthSpec, RelativeEpoch};
+
+pub mod justification_and_finalization;
+pub mod participation_record_updates;
+pub mod rewards_and_penalties;
+
+pub fn process_epoch<T: EthSpec>(
+    state: &mut BeaconState<T>,
+    spec: &ChainSpec,
+) -> Result<EpochProcessingSummary, Error> {
+    // Ensure the committee caches are built.
+    state.build_committee_cache(RelativeEpoch::Previous, spec)?;
+    state.build_committee_cache(RelativeEpoch::Current, spec)?;
+    state.build_committee_cache(RelativeEpoch::Next, spec)?;
+
+    // Load the struct we use to assign validators into sets based on their participation.
+    //
+    // E.g., attestation in the previous epoch, attested to the head, etc.
+    let mut validator_statuses = ValidatorStatuses::new(state, spec)?;
+    validator_statuses.process_attestations(&state)?;
+
+    // Justification and finalization.
+    process_justification_and_finalization(state, &validator_statuses.total_balances, spec)?;
+
+    // Rewards and Penalties.
+    process_rewards_and_penalties(state, &mut validator_statuses, spec)?;
+
+    // Registry Updates.
+    process_registry_updates(state, spec)?;
+
+    // Slashings.
+    process_slashings(
+        state,
+        validator_statuses.total_balances.current_epoch(),
+        spec.proportional_slashing_multiplier,
+        spec,
+    )?;
+
+    // Reset eth1 data votes.
+    process_eth1_data_reset(state)?;
+
+    // Update effective balances with hysteresis (lag).
+    process_effective_balance_updates(state, spec)?;
+
+    // Reset slashings
+    process_slashings_reset(state)?;
+
+    // Set randao mix
+    process_randao_mixes_reset(state)?;
+
+    // Set historical root accumulator
+    process_historical_roots_update(state)?;
+
+    // Rotate current/previous epoch attestations
+    process_participation_record_updates(state)?;
+
+    // Rotate the epoch caches to suit the epoch transition.
+    state.advance_caches()?;
+
+    Ok(EpochProcessingSummary {
+        total_balances: validator_statuses.total_balances,
+        statuses: validator_statuses.statuses,
+    })
+}