Converted the Beacon API service to Futures

- Added SSZ encode for HeadResponse
 - Converted all of the /beacon/ endpoints to return BoxFut instead of ApiResult
 - Wrapped all of the '?'s in a new macro try_future!()
 - Copied the try macro to try_future, so that a boxed future can easily be returned.
 - Replaced all of the response serializations to use the new success_response

beacon_node/rest_api/src/helpers.rs

@@ -227,7 +227,7 @@ pub fn get_beacon_chain_from_request<T: BeaconChainTypes + 'static>(
     let beacon_chain = req
         .extensions()
         .get::<Arc<BeaconChain<T>>>()
-        .ok_or_else(|| ApiError::ServerError("Beacon chain extension missing".into()))?;
+        .expect("BeaconChain extension must be there, because we put it there.");
     let mut head_state = beacon_chain
         .state_now()
         .map_err(|e| ApiError::ServerError(format!("Unable to get current BeaconState {:?}", e)))?;