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

lcli/src/transition_blocks.rs

@@ -74,7 +74,7 @@ use eth2::{
 use ssz::Encode;
 use state_processing::{
     block_signature_verifier::BlockSignatureVerifier, per_block_processing, per_slot_processing,
-    BlockSignatureStrategy, ConsensusContext, StateProcessingStrategy, VerifyBlockRoot,
+    BlockSignatureStrategy, ConsensusContext, EpochCache, StateProcessingStrategy, VerifyBlockRoot,
 };
 use std::borrow::Cow;
 use std::fs::File;