Changes tries: support forks (#950)

* forks support in changes trie storage

* moved convert_hash to primitives

* optimize ChangesTrieRootsStorage::root when anchor is canonicalized

substrate/srml/system/src/lib.rs

@@ -284,7 +284,7 @@ impl<T: Trait> Module<T> {
 		let mut digest = <Digest<T>>::take();
 		let extrinsics_root = <ExtrinsicsRoot<T>>::take();
 		let storage_root = T::Hashing::storage_root();
-		let storage_changes_root = T::Hashing::storage_changes_root(number.as_());
+		let storage_changes_root = T::Hashing::storage_changes_root(parent_hash, number.as_() - 1);
 
 		// we can't compute changes trie root earlier && put it to the Digest
 		// because it will include all currently existing temporaries