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/src/config.rs

@@ -107,6 +107,10 @@ pub fn get_config<E: EthSpec>(
         client_config.http_api.allow_origin = Some(allow_origin.to_string());
     }
 
+    if cli_args.is_present("http-disable-legacy-spec") {
+        client_config.http_api.serve_legacy_spec = false;
+    }
+
     /*
      * Prometheus metrics HTTP server
      */