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/slashings.rs

@@ -0,0 +1,42 @@
+use crate::per_epoch_processing::Error;
+use safe_arith::{SafeArith, SafeArithIter};
+use types::{BeaconState, BeaconStateError, ChainSpec, EthSpec, Unsigned};
+
+/// Process slashings.
+pub fn process_slashings<T: EthSpec>(
+    state: &mut BeaconState<T>,
+    total_balance: u64,
+    slashing_multiplier: u64,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    let epoch = state.current_epoch();
+    let sum_slashings = state.get_all_slashings().iter().copied().safe_sum()?;
+
+    let adjusted_total_slashing_balance =
+        std::cmp::min(sum_slashings.safe_mul(slashing_multiplier)?, total_balance);
+
+    let (validators, balances) = state.validators_and_balances_mut();
+    for (index, validator) in validators.iter().enumerate() {
+        if validator.slashed
+            && epoch.safe_add(T::EpochsPerSlashingsVector::to_u64().safe_div(2)?)?
+                == validator.withdrawable_epoch
+        {
+            let increment = spec.effective_balance_increment;
+            let penalty_numerator = validator
+                .effective_balance
+                .safe_div(increment)?
+                .safe_mul(adjusted_total_slashing_balance)?;
+            let penalty = penalty_numerator
+                .safe_div(total_balance)?
+                .safe_mul(increment)?;
+
+            // Equivalent to `decrease_balance(state, index, penalty)`, but avoids borrowing `state`.
+            let balance = balances
+                .get_mut(index)
+                .ok_or(BeaconStateError::BalancesOutOfBounds(index))?;
+            *balance = balance.saturating_sub(penalty);
+        }
+    }
+
+    Ok(())
+}