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>

consensus/state_processing/src/per_block_processing/verify_deposit.rs

@@ -60,7 +60,7 @@ pub fn verify_deposit_merkle_proof<T: EthSpec>(
             &deposit.proof[..],
             spec.deposit_contract_tree_depth.safe_add(1)? as usize,
             deposit_index as usize,
-            state.eth1_data.deposit_root,
+            state.eth1_data().deposit_root,
         ),
         DepositInvalid::BadMerkleProof
     );