Altair consensus changes and refactors (#2279)

## Proposed Changes

Implement the consensus changes necessary for the upcoming Altair hard fork.

## Additional Info

This is quite a heavy refactor, with pivotal types like the `BeaconState` and `BeaconBlock` changing from structs to enums. This ripples through the whole codebase with field accesses changing to methods, e.g. `state.slot` => `state.slot()`.


Co-authored-by: realbigsean <seananderson33@gmail.com>

testing/remote_signer_test/src/local_signer_test_data.rs

@@ -42,7 +42,7 @@ impl LocalSignerTestData<BeaconBlock<E>> {
             &self.spec,
         );
 
-        signed_block.signature.to_string()
+        signed_block.signature().to_string()
     }
 }
 

testing/remote_signer_test/src/utils.rs

@@ -239,31 +239,39 @@ pub fn get_block<E: EthSpec>(seed: u64) -> BeaconBlock<E> {
     let mut block: BeaconBlock<E> = BeaconBlock::empty(spec);
     for _ in 0..E::MaxProposerSlashings::to_usize() {
         block
-            .body
-            .proposer_slashings
+            .body_mut()
+            .proposer_slashings_mut()
             .push(proposer_slashing.clone())
             .unwrap();
     }
     for _ in 0..E::MaxDeposits::to_usize() {
-        block.body.deposits.push(deposit.clone()).unwrap();
+        block
+            .body_mut()
+            .deposits_mut()
+            .push(deposit.clone())
+            .unwrap();
     }
     for _ in 0..E::MaxVoluntaryExits::to_usize() {
         block
-            .body
-            .voluntary_exits
+            .body_mut()
+            .voluntary_exits_mut()
             .push(signed_voluntary_exit.clone())
             .unwrap();
     }
     for _ in 0..E::MaxAttesterSlashings::to_usize() {
         block
-            .body
-            .attester_slashings
+            .body_mut()
+            .attester_slashings_mut()
             .push(attester_slashing.clone())
             .unwrap();
     }
 
     for _ in 0..E::MaxAttestations::to_usize() {
-        block.body.attestations.push(attestation.clone()).unwrap();
+        block
+            .body_mut()
+            .attestations_mut()
+            .push(attestation.clone())
+            .unwrap();
     }
     block
 }