Check storage_key for validity (#2316)

* Intro `ChildStorageKey` for checked child keys

* Get rid of Into in Externalities trait

* Use Cow in ChildStorageKey

* Fix tests for state-machine.

* Clean

* child_storage_root always return a value

* Don't return Option from Ext::child_storage_root

* Return 42 in child_storage_root

* Return CHILD_STORAGE_KEY_PREFIX from trie id gen

* Bump spec and impl version.

* Require `:default:` in `is_child_trie_key_valid`

* Add `default:` prefix.

* Introduce `into_owned` for `ChildStorageKey`.

* Add documentation.

* Fix state-machine tests

* Remove outdated TODO

I check out with Emeric and he is ok with that

* child_storage_root is infailable

* Nit

* Move assert after check.

* Apply suggestions from @DemiMarie-parity

Co-Authored-By: pepyakin <s.pepyakin@gmail.com>

* Formatting nit in core/executor/src/wasm_executor.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Formatting nits from @thiolliere

substrate/core/trie/src/lib.rs

@@ -137,9 +137,23 @@ where
 	)
 }
 
-/// Determine whether a child trie key is valid. `child_trie_root` and `child_delta_trie_root` can panic if invalid value is provided to them.
-pub fn is_child_trie_key_valid<H: Hasher>(_storage_key: &[u8]) -> bool {
-	true
+/// Determine whether a child trie key is valid.
+///
+/// For now, the only valid child trie key is `:child_storage:default:`.
+///
+/// `child_trie_root` and `child_delta_trie_root` can panic if invalid value is provided to them.
+pub fn is_child_trie_key_valid<H: Hasher>(storage_key: &[u8]) -> bool {
+	use substrate_primitives::storage::well_known_keys;
+	let has_right_prefix = storage_key.starts_with(b":child_storage:default:");
+	if has_right_prefix {
+		// This is an attempt to catch a change of `is_child_storage_key`, which
+		// just checks if the key has prefix `:child_storage:` at the moment of writing.
+		debug_assert!(
+			well_known_keys::is_child_storage_key(&storage_key),
+			"`is_child_trie_key_valid` is a subset of `is_child_storage_key`",
+		);
+	}
+	has_right_prefix
 }
 
 /// Determine the default child trie root.