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/executor/src/wasm_runtime.rs

@@ -304,7 +304,7 @@ pub fn create_wasm_runtime_with_code(
 			//
 			// We drop the cache_path here to silence warnings that cache_path is not used if
 			// compiling without the `wasmtime` flag.
-			drop(cache_path);
+			let _ = cache_path;
 
 			sc_executor_wasmi::create_runtime(
 				blob,

substrate/client/network/src/block_request_handler.rs

@@ -85,13 +85,14 @@ struct SeenRequestsKey<B: BlockT> {
 	support_multiple_justifications: bool,
 }
 
+#[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);
 		self.max_blocks.hash(state);
 		self.direction.hash(state);
 		self.attributes.hash(state);
-
+		self.support_multiple_justifications.hash(state);
 		match self.from {
 			BlockId::Hash(h) => h.hash(state),
 			BlockId::Number(n) => n.hash(state),

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);