Update to spec v0.9.1 (#597)

* Update to spec v0.9.0

* Update to v0.9.1

* Bump spec tags for v0.9.1

* Formatting, fix CI failures

* Resolve accidental KeyPair merge conflict

* Document new BeaconState functions

* Fix incorrect cache drops in `advance_caches`

* Update fork choice for v0.9.1

* Clean up some FIXMEs

* Fix a few docs/logs

beacon_node/beacon_chain/src/builder.rs

@@ -231,7 +231,12 @@ where
             .genesis_block_root
             .ok_or_else(|| "fork_choice_backend requires a genesis_block_root")?;
 
-        self.fork_choice = Some(ForkChoice::new(store, backend, genesis_block_root));
+        self.fork_choice = Some(ForkChoice::new(
+            store,
+            backend,
+            genesis_block_root,
+            self.spec.genesis_slot,
+        ));
 
         Ok(self)
     }