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/common/base.rs

@@ -0,0 +1,19 @@
+use integer_sqrt::IntegerSquareRoot;
+use safe_arith::SafeArith;
+use types::*;
+
+/// Returns the base reward for some validator.
+pub fn get_base_reward<T: EthSpec>(
+    state: &BeaconState<T>,
+    index: usize,
+    // Should be == get_total_active_balance(state, spec)
+    total_active_balance: u64,
+    spec: &ChainSpec,
+) -> Result<u64, BeaconStateError> {
+    state
+        .get_effective_balance(index)?
+        .safe_mul(spec.base_reward_factor)?
+        .safe_div(total_active_balance.integer_sqrt())?
+        .safe_div(spec.base_rewards_per_epoch)
+        .map_err(Into::into)
+}