Reduce usage of Blake2Hasher (#5132)

This reduces the usage of `Blake2Hasher` in the code base and replaces
it with `BlakeTwo256`. The most important change is the removal of the
custom extern function for `Blake2Hasher`. The runtime `Hash` trait is
now also simplified and directly requires that the implementing type
implements `Hashable`.

substrate/client/service/src/builder.rs

@@ -39,7 +39,7 @@ use sc_network::{NetworkService, NetworkStateInfo};
 use parking_lot::{Mutex, RwLock};
 use sp_runtime::generic::BlockId;
 use sp_runtime::traits::{
-	Block as BlockT, NumberFor, SaturatedConversion, HasherFor, UniqueSaturatedInto,
+	Block as BlockT, NumberFor, SaturatedConversion, HashFor, UniqueSaturatedInto,
 };
 use sp_api::ProvideRuntimeApi;
 use sc_executor::{NativeExecutor, NativeExecutionDispatch};
@@ -159,19 +159,19 @@ pub type TLightClient<TBl, TRtApi, TExecDisp> = Client<
 /// Light client backend type.
 pub type TLightBackend<TBl> = sc_client::light::backend::Backend<
 	sc_client_db::light::LightStorage<TBl>,
-	HasherFor<TBl>,
+	HashFor<TBl>,
 >;
 
 /// Light call executor type.
 pub type TLightCallExecutor<TBl, TExecDisp> = sc_client::light::call_executor::GenesisCallExecutor<
 	sc_client::light::backend::Backend<
 		sc_client_db::light::LightStorage<TBl>,
-		HasherFor<TBl>
+		HashFor<TBl>
 	>,
 	sc_client::LocalCallExecutor<
 		sc_client::light::backend::Backend<
 			sc_client_db::light::LightStorage<TBl>,
-			HasherFor<TBl>
+			HashFor<TBl>
 		>,
 		NativeExecutor<TExecDisp>
 	>,