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/proving_backend.rs

@@ -174,6 +174,10 @@ impl<'a, S, H> Backend<H> for ProvingBackend<'a, S, H>
 		self.backend.for_keys_with_prefix(prefix, f)
 	}
 
+	fn for_key_values_with_prefix<F: FnMut(&[u8], &[u8])>(&self, prefix: &[u8], f: F) {
+		self.backend.for_key_values_with_prefix(prefix, f)
+	}
+
 	fn for_child_keys_with_prefix<F: FnMut(&[u8])>(&self, storage_key: &[u8], prefix: &[u8], f: F) {
 		self.backend.for_child_keys_with_prefix(storage_key, prefix, f)
 	}