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/frame/benchmarking/src/tests.rs

@@ -21,10 +21,7 @@
 use super::*;
 use codec::Decode;
 use sp_std::prelude::*;
-use sp_runtime::{
-	traits::{Dispatchable, BlakeTwo256, IdentityLookup},
-	testing::{H256, Header},
-};
+use sp_runtime::{traits::{BlakeTwo256, IdentityLookup}, testing::{H256, Header}};
 use frame_support::{dispatch::DispatchResult, decl_module, impl_outer_origin};
 use frame_system::{RawOrigin, ensure_signed, ensure_none};
 
@@ -166,4 +163,4 @@ fn benchmarks_macro_works_for_non_dispatchable() {
 	).expect("failed to create closure");
 
 	assert_eq!(closure(), Ok(()));
-}
+}