add clippy to CI (#9694)

* Focus on correctness lints. This relies on a compiler patch that landed 8th July 2021.
If people are using an earlier version of the compiler everything will still work unless they try and run clippy.

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Denis Pisarev <denis.pisarev@parity.io>

substrate/client/network/src/state_request_handler.rs

@@ -78,6 +78,7 @@ struct SeenRequestsKey<B: BlockT> {
 	start: Vec<u8>,
 }
 
+#[allow(clippy::derive_hash_xor_eq)]
 impl<B: BlockT> Hash for SeenRequestsKey<B> {
 	fn hash<H: Hasher>(&self, state: &mut H) {
 		self.peer.hash(state);