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/macros.rs

@@ -0,0 +1,13 @@
+macro_rules! try_future {
+    ($expr:expr) => {
+        match $expr {
+            core::result::Result::Ok(val) => val,
+            core::result::Result::Err(err) => {
+                return Box::new(futures::future::err(std::convert::From::from(err)))
+            }
+        }
+    };
+    ($expr:expr,) => {
+        $crate::try_future!($expr)
+    };
+}