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/state_id.rs

@@ -57,7 +57,7 @@ impl StateId {
         &self,
         chain: &BeaconChain<T>,
     ) -> Result<Fork, warp::Rejection> {
-        self.map_state(chain, |state| Ok(state.fork))
+        self.map_state(chain, |state| Ok(state.fork()))
     }
 
     /// Return the `BeaconState` identified by `self`.