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/authority-discovery/src/tests.rs

@@ -215,7 +215,7 @@ impl ApiExt<Block> for RuntimeApi {
 	fn into_storage_changes(
 		&self,
 		_: &Self::StateBackend,
-		_: Option<&sp_api::ChangesTrieState<sp_api::HasherFor<Block>, sp_api::NumberFor<Block>>>,
+		_: Option<&sp_api::ChangesTrieState<sp_api::HashFor<Block>, sp_api::NumberFor<Block>>>,
 		_: <Block as sp_api::BlockT>::Hash,
 	) -> std::result::Result<sp_api::StorageChanges<Self::StateBackend, Block>, String>
 		where Self: Sized