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,

beacon_node/store/src/hdiff.rs

@@ -331,7 +331,7 @@ mod tests {
 
         let xor_diff = XorDiff::compute(&x_values, &y_values).unwrap();
 
-        let mut y_from_xor = x_values.clone();
+        let mut y_from_xor = x_values;
         xor_diff.apply(&mut y_from_xor).unwrap();
 
         assert_eq!(y_values, y_from_xor);

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(),
         )
     }
 

beacon_node/store/src/leveldb_store.rs

@@ -183,7 +183,7 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
     }
 
     fn iter_column_from<K: Key>(&self, column: DBColumn, from: &[u8]) -> ColumnIter<K> {
-        let start_key = BytesKey::from_vec(get_key_for_col(column.into(), &from));
+        let start_key = BytesKey::from_vec(get_key_for_col(column.into(), from));
 
         let iter = self.db.iter(self.read_options());
         iter.seek(&start_key);