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/client.rs

@@ -129,7 +129,7 @@ impl<H> PrePostHeader<H> {
 /// Create an instance of in-memory client.
 pub fn new_in_mem<E, Block, S, RA>(
 	executor: E,
-	genesis_storage: S,
+	genesis_storage: &S,
 	keystore: Option<sp_core::traits::BareCryptoStorePtr>,
 ) -> sp_blockchain::Result<Client<
 	in_mem::Backend<Block>,
@@ -149,7 +149,7 @@ pub fn new_in_mem<E, Block, S, RA>(
 pub fn new_with_backend<B, E, Block, S, RA>(
 	backend: Arc<B>,
 	executor: E,
-	build_genesis_storage: S,
+	build_genesis_storage: &S,
 	keystore: Option<sp_core::traits::BareCryptoStorePtr>,
 ) -> sp_blockchain::Result<Client<B, LocalCallExecutor<B, E>, Block, RA>>
 	where
@@ -187,7 +187,7 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 	pub fn new<S: BuildStorage>(
 		backend: Arc<B>,
 		executor: E,
-		build_genesis_storage: S,
+		build_genesis_storage: &S,
 		fork_blocks: ForkBlocks<Block>,
 		bad_blocks: BadBlocks<Block>,
 		execution_extensions: ExecutionExtensions<Block>,

substrate/client/src/lib.rs

@@ -65,7 +65,7 @@
 //! 		NativeExecutor::<LocalExecutor>::new(WasmExecutionMethod::Interpreted, None),
 //!		),
 //! 	// This parameter provides the storage for the chain genesis.
-//! 	<Storage>::default(),
+//! 	&<Storage>::default(),
 //! 	Default::default(),
 //! 	Default::default(),
 //! 	Default::default(),

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<