Fetching changes proof from remote nodes (#769)

* changes_trie

* changs_trie: continue

* changes_trie: adding tests

* fixed TODO

* removed obsolete ExtrinsicChanges

* encodable ChangesTrieConfiguration

* removed polkadot fle

* fixed grumbles

* ext_storage_changes_root returns u32

* moved changes trie root to digest

* removed commented code

* read storage values from native code

* fixed grumbles

* fixed grumbles

* missing comma

* key changes proof generation + query

* fix grumbles

* check that changes trie config is not changed by block.finalize()

* fixed changes trie config check

substrate/core/test-runtime/src/lib.rs

@@ -141,6 +141,14 @@ pub fn run_tests(mut input: &[u8]) -> Vec<u8> {
 	[stxs.len() as u8].encode()
 }
 
+/// Changes trie configuration (optionally) used in tests.
+pub fn changes_trie_config() -> primitives::ChangesTrieConfiguration {
+	primitives::ChangesTrieConfiguration {
+		digest_interval: 4,
+		digest_levels: 2,
+	}
+}
+
 pub mod api {
 	use system;
 	impl_stubs!(