Return number of keys removed when calling storage_kill on child trie (#8166)

* Initial piping of returning amount of keys killed

* One more test for `None` limit

* forgot to update

* fix return value

* use version 3

* Update to return `KillOutcome`

* Update name to KillChildStorageResult

substrate/primitives/tasks/src/async_externalities.rs

@@ -119,7 +119,7 @@ impl Externalities for AsyncExternalities {
 		&mut self,
 		_child_info: &ChildInfo,
 		_limit: Option<u32>,
-	) -> bool {
+	) -> (bool, u32) {
 		panic!("`kill_child_storage`: should not be used in async externalities!")
 	}