Take genesis_storage by ref (#4617)

Instead of having these weird implementation of `BuildStorage for
&ChainSpec` we should just take the `genesis_storage` by ref. The
`BuildStorage` trait changed some time ago to take a self ref anyway,
instead of a self value.

Also fixes warnings in frame-staking

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

@@ -190,7 +190,7 @@ impl<Executor, Backend, G: GenesisInit> TestClientBuilder<Executor, Backend, G>
 		let client = sc_client::Client::new(
 			self.backend.clone(),
 			executor,
-			storage,
+			&storage,
 			Default::default(),
 			Default::default(),
 			ExecutionExtensions::new(