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/bin/node/executor/benches/bench.rs

@@ -23,12 +23,13 @@ use node_runtime::{
 };
 use node_runtime::constants::currency::*;
 use node_testing::keyring::*;
-use sp_core::{Blake2Hasher, NativeOrEncoded, NeverNativeValue};
+use sp_core::{NativeOrEncoded, NeverNativeValue};
 use sp_core::storage::well_known_keys;
 use sp_core::traits::{CodeExecutor, RuntimeCode};
 use frame_support::Hashable;
 use sp_state_machine::TestExternalities as CoreTestExternalities;
 use sc_executor::{NativeExecutor, RuntimeInfo, WasmExecutionMethod, Externalities};
+use sp_runtime::traits::BlakeTwo256;
 
 criterion_group!(benches, bench_execute_block);
 criterion_main!(benches);
@@ -54,7 +55,7 @@ fn sign(xt: CheckedExtrinsic) -> UncheckedExtrinsic {
 	node_testing::keyring::sign(xt, VERSION, GENESIS_HASH)
 }
 
-fn new_test_ext(genesis_config: &GenesisConfig) -> TestExternalities<Blake2Hasher> {
+fn new_test_ext(genesis_config: &GenesisConfig) -> TestExternalities<BlakeTwo256> {
 	let mut test_ext = TestExternalities::new_with_code(
 		COMPACT_CODE,
 		genesis_config.build_storage().unwrap(),
@@ -76,7 +77,7 @@ fn construct_block<E: Externalities>(
 	let extrinsics = extrinsics.into_iter().map(sign).collect::<Vec<_>>();
 
 	// calculate the header fields that we can.
-	let extrinsics_root = Layout::<Blake2Hasher>::ordered_trie_root(
+	let extrinsics_root = Layout::<BlakeTwo256>::ordered_trie_root(
 		extrinsics.iter().map(Encode::encode)
 	).to_fixed_bytes()
 		.into();