Introduce original_storage and original_storage_hash (#2211)

Both functions will ignore any overlayed changes and access the backend
directly.

substrate/core/state-machine/src/basic.rs

@@ -111,6 +111,10 @@ impl<H: Hasher> Externalities<H> for BasicExternalities where H::Out: Ord + Heap
 		}
 	}
 
+	fn original_storage(&self, key: &[u8]) -> Option<Vec<u8>> {
+		Externalities::<H>::storage(self, key)
+	}
+
 	fn child_storage(&self, _storage_key: &[u8], _key: &[u8]) -> Option<Vec<u8>> {
 		None
 	}