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

testing/ef_tests/src/cases/sanity_blocks.rs

@@ -137,7 +137,7 @@ impl<E: EthSpec> Case for SanityBlocks<E> {
             post.build_all_committee_caches(spec).unwrap();
             post
         });
-        check_state_diff(&pre, &post)?;
+        check_state_diff(&pre, &post, spec)?;
         Ok(())
     }
 }