Tree hash caching and optimisations for Altair (#2459)

## Proposed Changes

Remove the remaining Altair `FIXME`s from consensus land.

1. Implement tree hash caching for the participation lists. This required some light type manipulation, including removing the `TreeHash` bound from `CachedTreeHash` which was purely descriptive.
2. Plumb the proposer index through Altair attestation processing, to avoid calculating it for _every_ attestation (potentially 128ms on large networks). This duplicates some work from #2431, but with the aim of getting it in sooner, particularly for the Altair devnets.
3. Removes two FIXMEs related to `superstruct` and cloning, which are unlikely to be particularly detrimental and will be tracked here instead: https://github.com/sigp/superstruct/issues/5

testing/ef_tests/src/cases/operations.rs

@@ -67,13 +67,19 @@ impl<E: EthSpec> Operation<E> for Attestation<E> {
         state: &mut BeaconState<E>,
         spec: &ChainSpec,
     ) -> Result<(), BlockProcessingError> {
+        let proposer_index = state.get_beacon_proposer_index(state.slot(), spec)? as u64;
         match state {
             BeaconState::Base(_) => {
                 base::process_attestations(state, &[self.clone()], VerifySignatures::True, spec)
             }
-            BeaconState::Altair(_) => {
-                altair::process_attestation(state, self, 0, VerifySignatures::True, spec)
-            }
+            BeaconState::Altair(_) => altair::process_attestation(
+                state,
+                self,
+                0,
+                proposer_index,
+                VerifySignatures::True,
+                spec,
+            ),
         }
     }
 }