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/observed_operations.rs

@@ -1,7 +1,6 @@
 use derivative::Derivative;
 use smallvec::{smallvec, SmallVec};
-use ssz::{Decode, Encode};
-use state_processing::{SigVerifiedOp, VerifyOperation, VerifyOperationAt};
+use state_processing::{SigVerifiedOp, TransformPersist, VerifyOperation, VerifyOperationAt};
 use std::collections::HashSet;
 use std::marker::PhantomData;
 use types::{
@@ -34,7 +33,7 @@ pub struct ObservedOperations<T: ObservableOperation<E>, E: EthSpec> {
 
 /// Was the observed operation new and valid for further processing, or a useless duplicate?
 #[derive(Debug, PartialEq, Eq, Clone)]
-pub enum ObservationOutcome<T: Encode + Decode, E: EthSpec> {
+pub enum ObservationOutcome<T: TransformPersist, E: EthSpec> {
     New(SigVerifiedOp<T, E>),
     AlreadyKnown,
 }
@@ -62,12 +61,12 @@ impl<E: EthSpec> ObservableOperation<E> for ProposerSlashing {
 impl<E: EthSpec> ObservableOperation<E> for AttesterSlashing<E> {
     fn observed_validators(&self) -> SmallVec<[u64; SMALL_VEC_SIZE]> {
         let attestation_1_indices = self
-            .attestation_1
+            .attestation_1()
             .attesting_indices_iter()
             .copied()
             .collect::<HashSet<u64>>();
         let attestation_2_indices = self
-            .attestation_2
+            .attestation_2()
             .attesting_indices_iter()
             .copied()
             .collect::<HashSet<u64>>();