superstruct the AttesterSlashing (#5636)

* `superstruct` Attester Fork Variants

* Push a little further

* Deal with Encode / Decode of AttesterSlashing

* not so sure about this..

* Stop Encode/Decode Bounds from Propagating Out

* Tons of Changes..

* More Conversions to AttestationRef

* Add AsReference trait (#15)

* Add AsReference trait

* Fix some snafus

* Got it Compiling! :D

* Got Tests Building

* Get beacon chain tests compiling

---------

Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/http_api/src/lib.rs

@@ -1635,7 +1635,12 @@ pub fn serve<T: BeaconChainTypes>(
                     let (block, execution_optimistic, finalized) =
                         block_id.blinded_block(&chain)?;
                     Ok(api_types::GenericResponse::from(
-                        block.message().body().attestations().clone(),
+                        block
+                            .message()
+                            .body()
+                            .attestations()
+                            .map(|att| att.clone_as_attestation())
+                            .collect::<Vec<_>>(),
                     )
                     .add_execution_optimistic_finalized(execution_optimistic, finalized))
                 })
@@ -1833,7 +1838,7 @@ pub fn serve<T: BeaconChainTypes>(
                     chain
                         .validator_monitor
                         .read()
-                        .register_api_attester_slashing(&slashing);
+                        .register_api_attester_slashing(slashing.to_ref());
 
                     if let ObservationOutcome::New(slashing) = outcome {
                         publish_pubsub_message(