Child trie storage proof (#2433)

* proof on child trie

* higher level api for child storage proof

* boilerplate for proof from light fetch

* actually check proof on light fetch

* Do not break former encoding

* tabify

* tabify2

* Add child trie root tx to full_storage_root transaction.

* Shorten long lines.

* Temp rename for audit

* Make full_storage a trait method

* Name back and replace some code with full_storage where it looks fine.

* fix indentations, remove unused import

* flush child root to top when calculated

* impl +1

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

@@ -303,6 +303,13 @@ impl<Block: BlockT> client::light::fetcher::Fetcher<Block> for LightFetcher {
 		unimplemented!("not (yet) used in tests")
 	}
 
+	fn remote_read_child(
+		&self,
+		_request: client::light::fetcher::RemoteReadChildRequest<Block::Header>,
+	) -> Self::RemoteReadResult {
+		unimplemented!("not (yet) used in tests")
+	}
+
 	fn remote_call(
 		&self,
 		_request: client::light::fetcher::RemoteCallRequest<Block::Header>,