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/io/src/lib.rs

@@ -117,6 +117,11 @@ pub trait Storage {
 		Externalities::clear_prefix(*self, prefix)
 	}
 
+	/// Append to storage item (assumes it is in "Vec" format).
+	fn append(&mut self, key: &[u8], value: Vec<u8>) {
+		self.storage_append(key.to_vec(), value);
+	}
+
 	/// "Commit" all existing operations and compute the resulting storage root.
 	///
 	/// The hashing algorithm is defined by the `Block`.