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/client/src/light/mod.rs

@@ -56,7 +56,7 @@ pub fn new_light_backend<B, S>(blockchain: Arc<Blockchain<S>>) -> Arc<Backend<S,
 /// Create an instance of light client.
 pub fn new_light<B, S, GS, RA, E>(
 	backend: Arc<Backend<S, HasherFor<B>>>,
-	genesis_storage: GS,
+	genesis_storage: &GS,
 	code_executor: E,
 ) -> ClientResult<
 		Client<