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_exit.rs

@@ -52,7 +52,7 @@ fn verify_exit_parametric<T: EthSpec>(
     let exit = &signed_exit.message;
 
     let validator = state
-        .validators
+        .validators()
         .get(exit.validator_index as usize)
         .ok_or_else(|| error(ExitInvalid::ValidatorUnknown(exit.validator_index)))?;