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/primitives/state-machine/src/in_memory_backend.rs

@@ -362,11 +362,12 @@ impl<H: Hasher> Backend<H> for InMemory<H> where H::Out: Codec {
 #[cfg(test)]
 mod tests {
 	use super::*;
+	use sp_runtime::traits::BlakeTwo256;
 
 	/// Assert in memory backend with only child trie keys works as trie backend.
 	#[test]
 	fn in_memory_with_child_trie_only() {
-		let storage = InMemory::<sp_core::Blake2Hasher>::default();
+		let storage = InMemory::<BlakeTwo256>::default();
 		let child_info = OwnedChildInfo::new_default(b"unique_id_1".to_vec());
 		let mut storage = storage.update(
 			vec![(