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/beacon_chain/src/block_reward.rs

@@ -27,10 +27,12 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         let split_attestations = block
             .body()
             .attestations()
-            .iter()
             .map(|att| {
                 let attesting_indices = get_attesting_indices_from_state(state, att)?;
-                Ok(SplitAttestation::new(att.clone(), attesting_indices))
+                Ok(SplitAttestation::new(
+                    att.clone_as_attestation(),
+                    attesting_indices,
+                ))
             })
             .collect::<Result<Vec<_>, BeaconChainError>>()?;
 
@@ -86,7 +88,6 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             block
                 .body()
                 .attestations()
-                .iter()
                 .map(|a| a.data().clone())
                 .collect()
         } else {