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/client/src/error.rs

@@ -100,6 +100,18 @@ error_chain! {
 			display("Error decoding call result of {}", method)
 		}
 
+		/// Changes tries are not supported.
+		ChangesTriesNotSupported {
+			description("changes tries are not supported"),
+			display("Changes tries are not supported by the runtime"),
+		}
+
+		/// Key changes query has failed.
+		ChangesTrieAccessFailed(e: String) {
+			description("invalid changes proof"),
+			display("Failed to check changes proof: {}", e),
+		}
+
 		/// Last finalized block not parent of current.
 		NonSequentialFinalization(s: String) {
 			description("Did not finalize blocks in sequential order."),