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/client/src/client.rs

@@ -52,7 +52,7 @@ use primitives::storage::well_known_keys;
 use parity_codec::{Encode, Decode};
 use state_machine::{
 	DBValue, Backend as StateBackend, CodeExecutor, ChangesTrieAnchorBlockId,
-	ExecutionStrategy, ExecutionManager, prove_read,
+	ExecutionStrategy, ExecutionManager, prove_read, prove_child_read,
 	ChangesTrieRootsStorage, ChangesTrieStorage,
 	key_changes, key_changes_proof, OverlayedChanges, NeverOffchainExt,
 };
@@ -374,6 +374,20 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 				.map_err(Into::into))
 	}
 
+	/// Reads child storage value at a given block + storage_key + key, returning
+	/// read proof.
+	pub fn read_child_proof(
+		&self,
+		id: &BlockId<Block>,
+		storage_key: &[u8],
+		key: &[u8]
+	) -> error::Result<Vec<Vec<u8>>> {
+		self.state_at(id)
+			.and_then(|state| prove_child_read(state, storage_key, key)
+				.map(|(_, proof)| proof)
+				.map_err(Into::into))
+	}
+
 	/// Execute a call to a contract on top of state in a block of given hash
 	/// AND returning execution proof.
 	///