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/state-machine/src/ext.rs

@@ -391,7 +391,7 @@ where
 		&mut self,
 		child_info: &ChildInfo,
 		limit: Option<u32>,
-	) -> bool {
+	) -> (bool, u32) {
 		trace!(target: "state", "{:04x}: KillChild({})",
 			self.id,
 			HexDisplay::from(&child_info.storage_key()),
@@ -399,9 +399,9 @@ where
 		let _guard = guard();
 		self.mark_dirty();
 		self.overlay.clear_child_storage(child_info);
+		let mut num_deleted: u32 = 0;
 
 		if let Some(limit) = limit {
-			let mut num_deleted: u32 = 0;
 			let mut all_deleted = true;
 			self.backend.apply_to_child_keys_while(child_info, |key| {
 				if num_deleted == limit {
@@ -417,13 +417,14 @@ where
 				self.overlay.set_child_storage(child_info, key.to_vec(), None);
 				true
 			});
-			all_deleted
+			(all_deleted, num_deleted)
 		} else {
 			self.backend.apply_to_child_keys_while(child_info, |key| {
+				num_deleted = num_deleted.saturating_add(1);
 				self.overlay.set_child_storage(child_info, key.to_vec(), None);
 				true
 			});
-			true
+			(true, num_deleted)
 		}
 	}