Implement change trie for child trie. (#3122)

* Initial implementation, some redundancy is awkward and there is
some useless computation (but there is a pending pr for that).
Next are tests.

* Minimal tests and fix extend child.

* implement iterator for change child trie.

* prune child trie.

* Fix pruning test.

* bump spec version.

* Avoid empty child trie (could also be checked before)

* tabs.

* Fix child digest overriding each others.

* Restore doc deleted on merge.

* Check correct child value on extrinsics build.

* Revert runtime version update.

substrate/core/state-machine/src/backend.rs

@@ -72,7 +72,14 @@ pub trait Backend<H: Hasher> {
 
 	/// Retrieve all entries keys which start with the given prefix and
 	/// call `f` for each of those keys.
-	fn for_keys_with_prefix<F: FnMut(&[u8])>(&self, prefix: &[u8], f: F);
+	fn for_keys_with_prefix<F: FnMut(&[u8])>(&self, prefix: &[u8], mut f: F) {
+		self.for_key_values_with_prefix(prefix, |k, _v| f(k))
+	}
+
+	/// Retrieve all entries keys and values of which start with the given prefix and
+	/// call `f` for each of those keys.
+	fn for_key_values_with_prefix<F: FnMut(&[u8], &[u8])>(&self, prefix: &[u8], f: F);
+
 
 	/// Retrieve all child entries keys which start with the given prefix and
 	/// call `f` for each of those keys.
@@ -321,6 +328,11 @@ impl<H: Hasher> Backend<H> for InMemory<H> {
 		self.inner.get(&None).map(|map| map.keys().filter(|key| key.starts_with(prefix)).map(|k| &**k).for_each(f));
 	}
 
+	fn for_key_values_with_prefix<F: FnMut(&[u8], &[u8])>(&self, prefix: &[u8], mut f: F) {
+		self.inner.get(&None).map(|map| map.iter().filter(|(key, _val)| key.starts_with(prefix))
+			.for_each(|(k, v)| f(k, v)));
+	}
+
 	fn for_keys_in_child_storage<F: FnMut(&[u8])>(&self, storage_key: &[u8], mut f: F) {
 		self.inner.get(&Some(storage_key.to_vec())).map(|map| map.keys().for_each(|k| f(&k)));
 	}