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/contracts/src/lib.rs

@@ -254,7 +254,7 @@ where
 		let mut buf = Vec::new();
 		storage_root.using_encoded(|encoded| buf.extend_from_slice(encoded));
 		buf.extend_from_slice(code_hash.as_ref());
-		RawTombstoneContractInfo(Hasher::hash(&buf[..]), PhantomData)
+		RawTombstoneContractInfo(<Hasher as Hash>::hash(&buf[..]), PhantomData)
 	}
 }