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/chain-spec/src/chain_spec.rs

@@ -70,7 +70,7 @@ impl<G: RuntimeGenesis> GenesisSource<G> {
 	}
 }
 
-impl<'a, G: RuntimeGenesis, E> BuildStorage for &'a ChainSpec<G, E> {
+impl<G: RuntimeGenesis, E> BuildStorage for ChainSpec<G, E> {
 	fn build_storage(&self) -> Result<Storage, String> {
 		match self.genesis.resolve()? {
 			Genesis::Runtime(gc) => gc.build_storage(),