Address clippy lints in tree-states (#4414)

* Address some clippy lints

* Box errors to fix error size lint

* Add Default impl for Validator

* Address more clippy lints

* Re-implement `check_state_diff`

* Fix misc test compile errors

beacon_node/store/src/hot_cold_store.rs

@@ -1573,7 +1573,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
             (start_slot.as_u64()..=end_slot.as_u64())
                 .map(Slot::new)
                 .map(|slot| self.get_cold_blinded_block_by_slot(slot)),
-            |iter| iter.filter_map(|x| x).collect(),
+            |iter| iter.flatten().collect(),
         )
     }