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>

beacon_node/http_api/src/proposer_duties.rs

@@ -148,7 +148,7 @@ fn compute_and_cache_proposer_duties<T: BeaconChainTypes>(
             state.current_epoch(),
             dependent_root,
             indices.clone(),
-            state.fork,
+            state.fork(),
         )
         .map_err(BeaconChainError::from)
         .map_err(warp_utils::reject::beacon_chain_error)?;