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/beacon_chain/tests/store_tests.rs

@@ -28,8 +28,6 @@ use store::{
     HotColdDB, LevelDB, StoreConfig,
 };
 use tempfile::{tempdir, TempDir};
-use tokio::time::sleep;
-use tree_hash::TreeHash;
 use types::test_utils::{SeedableRng, XorShiftRng};
 use types::*;