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/db/src/lib.rs

@@ -137,6 +137,10 @@ impl<B: BlockT> StateBackend<Blake2Hasher> for RefTrackingState<B> {
 		self.state.for_keys_with_prefix(prefix, f)
 	}
 
+	fn for_key_values_with_prefix<F: FnMut(&[u8], &[u8])>(&self, prefix: &[u8], f: F) {
+		self.state.for_key_values_with_prefix(prefix, f)
+	}
+
 	fn for_keys_in_child_storage<F: FnMut(&[u8])>(&self, storage_key: &[u8], f: F) {
 		self.state.for_keys_in_child_storage(storage_key, f)
 	}

substrate/core/client/db/src/storage_cache.rs

@@ -536,6 +536,10 @@ impl<H: Hasher, S: StateBackend<H>, B: BlockT> StateBackend<H> for CachingState<
 		self.state.for_keys_with_prefix(prefix, f)
 	}
 
+	fn for_key_values_with_prefix<F: FnMut(&[u8], &[u8])>(&self, prefix: &[u8], f: F) {
+		self.state.for_key_values_with_prefix(prefix, f)
+	}
+
 	fn for_keys_in_child_storage<F: FnMut(&[u8])>(&self, storage_key: &[u8], f: F) {
 		self.state.for_keys_in_child_storage(storage_key, f)
 	}