Refactor OverlayedChanges (#5989)

* Hide internal structure of OverlayChanges

* Fix tests for OverlayChanges refactor

* Do not clone pending changes

Discarding prospective changes should be equivalent as a state machine
is not to be called with peding changes.

This will be replaced by a storage transaction that is rolled back before
executing the call the second time removing this constraint.

* Doc fixes

* Remove overlong line

* Revert "Do not clone pending changes"

This reverts commit 4799491f4ac16f8517287a0fcf4a3f84ad56f46e.

* Deduplicate chield tries returned from child_infos()

* Remove redundant type annotation

* Avoid changing the storage root in tests

* Preserve extrinsic indices in trie build test

* Swap order of comitted and prospective in fn child_infos

This is only for consistency and does not impact the result.

* Rename set_pending to replace_pending for clearity

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

@@ -136,21 +136,19 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> TestExternalities<H, N>
 
 	/// Return a new backend with all pending value.
 	pub fn commit_all(&self) -> InMemoryBackend<H> {
-		let top: Vec<_> = self.overlay.committed.top.clone().into_iter()
-			.chain(self.overlay.prospective.top.clone().into_iter())
-			.map(|(k, v)| (k, v.value)).collect();
+		let top: Vec<_> = self.overlay.changes(None)
+			.map(|(k, v)| (k.clone(), v.value().cloned()))
+			.collect();
 		let mut transaction = vec![(None, top)];
 
-		self.overlay.committed.children_default.clone().into_iter()
-			.chain(self.overlay.prospective.children_default.clone().into_iter())
-			.for_each(|(_storage_key, (map, child_info))| {
-				transaction.push((
-					Some(child_info),
-					map.into_iter()
-						.map(|(k, v)| (k, v.value))
-						.collect::<Vec<_>>(),
-				))
-			});
+		for child_info in self.overlay.child_infos() {
+			transaction.push((
+				Some(child_info.clone()),
+				self.overlay.changes(Some(child_info))
+					.map(|(k, v)| (k.clone(), v.value().cloned()))
+					.collect(),
+			))
+		}
 
 		self.backend.update(transaction)
 	}