Make ShardLevelState::from_existing return trie::Result

TrieError has H256, so Result<..., TrieError> takes large space even
when there is no error. trie::Result is defined as
Result<..., Box<TrieError>> to prevent the space waste.

Author: Seulgi Kim

core/src/state/shard_level.rs

@@ -66,9 +66,9 @@ impl<B: Backend + ShardBackend> ShardLevelState<B> {
     }
 
     /// Creates new state with existing state root
-    pub fn from_existing(db: B, root: H256, trie_factory: TrieFactory) -> Result<ShardLevelState<B>, TrieError> {
+    pub fn from_existing(db: B, root: H256, trie_factory: TrieFactory) -> trie::Result<ShardLevelState<B>> {
         if !db.as_hashdb().contains(&root) {
-            return Err(TrieError::InvalidStateRoot(root))
+            return Err(TrieError::InvalidStateRoot(root).into())
         }
 
         Ok(ShardLevelState {