Add append api and use it to deposit events (#5795)

* use append api to deposit events

* use optimized append

* one more optimization

* fix bug

* fix issues

* address review

substrate/primitives/state-machine/src/basic.rs

@@ -257,6 +257,16 @@ impl Externalities for BasicExternalities {
 		}
 	}
 
+	fn storage_append(
+		&mut self,
+		key: Vec<u8>,
+		value: Vec<u8>,
+	) {
+		let previous = self.storage(&key).unwrap_or_default();
+		let new = crate::ext::append_to_storage(previous, value).expect("Failed to append to storage");
+		self.place_storage(key.clone(), Some(new));
+	}
+
 	fn chain_id(&self) -> u64 { 42 }
 
 	fn storage_root(&mut self) -> Vec<u8> {