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/client/src/light/backend.rs

@@ -381,6 +381,10 @@ where
 		// whole state is not available on light node
 	}
 
+	fn for_key_values_with_prefix<A: FnMut(&[u8], &[u8])>(&self, _prefix: &[u8], _action: A) {
+		// whole state is not available on light node
+	}
+
 	fn for_keys_in_child_storage<A: FnMut(&[u8])>(&self, _storage_key: &[u8], _action: A) {
 		// whole state is not available on light node
 	}
@@ -461,6 +465,15 @@ where
 		}
 	}
 
+	fn for_key_values_with_prefix<A: FnMut(&[u8], &[u8])>(&self, prefix: &[u8], action: A) {
+		match *self {
+			OnDemandOrGenesisState::OnDemand(ref state) =>
+				StateBackend::<H>::for_key_values_with_prefix(state, prefix, action),
+			OnDemandOrGenesisState::Genesis(ref state) => state.for_key_values_with_prefix(prefix, action),
+		}
+	}
+
+
 	fn for_keys_in_child_storage<A: FnMut(&[u8])>(&self, storage_key: &[u8], action: A) {
 		match *self {
 			OnDemandOrGenesisState::OnDemand(ref state) =>