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>

testing/simulator/src/checks.rs

@@ -99,7 +99,7 @@ async fn verify_validator_count<E: EthSpec>(
                 .await
                 .map(|body| body.unwrap().data)
                 .map_err(|e| format!("Get state root via http failed: {:?}", e))?
-                .validators
+                .validators()
                 .len();
             validator_counts.push(vc);
         }