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>

crypto/bls/src/generic_signature_set.rs

@@ -74,7 +74,7 @@ where
 impl<'a, Pub, AggPub, Sig, AggSig> GenericSignatureSet<'a, Pub, AggPub, Sig, AggSig>
 where
     Pub: TPublicKey + Clone,
-    AggPub: TAggregatePublicKey + Clone,
+    AggPub: TAggregatePublicKey<Pub> + Clone,
     Sig: TSignature<Pub> + Clone,
     AggSig: TAggregateSignature<Pub, AggPub, Sig> + Clone,
 {