Change kv_store::delete to remove

Rust HashMap has remove function that returns Option.
I changed kv_store::delete function to make consistent with other code.

Author: majecty

core/src/ibc/context.rs

@@ -85,9 +85,11 @@ impl<'a> KVStore for TopLevelKVStore<'a> {
         self.state.update_ibc_data(&key, value.to_vec()).expect("Set in IBC KVStore").map(Bytes::from)
     }
 
-    fn delete(&mut self, path: Path) {
+    fn remove(&mut self, path: Path) -> Option<Bytes> {
+        let prev = self.get(path);
         let key = TopLevelKVStore::key(path);
         self.state.remove_ibc_data(&key);
+        prev
     }
 
     fn root(&self) -> H256 {

core/src/ibc/kv_store/mod.rs

@@ -24,7 +24,7 @@ pub trait KVStore {
     fn get(&self, path: Path) -> Option<Bytes>;
     fn contains_key(&self, path: Path) -> bool;
     fn insert(&mut self, path: Path, value: &[u8]) -> Option<Bytes>;
-    fn delete(&mut self, path: Path);
+    fn remove(&mut self, path: Path) -> Option<Bytes>;
     fn root(&self) -> H256;
     fn make_proof(&self, path: Path) -> (CryptoProofUnit, CryptoProof);
 }