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/forwards_iter.rs

@@ -234,7 +234,7 @@ impl<'a, E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>
                     // of the pre iterator.
                     None => {
                         let continuation_data = continuation_data.take();
-                        let start_slot = Slot::from(iter.limit);
+                        let start_slot = iter.limit;
 
                         *self = PostFinalizationLazy {
                             continuation_data,