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>

beacon_node/network/src/service/tests.rs

@@ -38,6 +38,7 @@ mod tests {
         let beacon_chain = Arc::new(
             BeaconChainHarness::new_with_store_config(
                 MinimalEthSpec,
+                None,
                 generate_deterministic_keypairs(8),
                 StoreConfig::default(),
             )