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>

validator_client/src/block_service.rs

@@ -289,8 +289,8 @@ impl<T: SlotClock + 'static, E: EthSpec> BlockService<T, E> {
         info!(
             log,
             "Successfully published block";
-            "deposits" => signed_block.message.body.deposits.len(),
-            "attestations" => signed_block.message.body.attestations.len(),
+            "deposits" => signed_block.message().body().deposits().len(),
+            "attestations" => signed_block.message().body().attestations().len(),
             "graffiti" => ?graffiti.map(|g| g.as_utf8_lossy()),
             "slot" => signed_block.slot().as_u64(),
         );