Enable check that Hash and PartialEq impls aren't different

This is a crude check, just seeing if Hash is derived and PartialEq is not, but it's
at least something.

Author: Susurrus

src/lib.rs

@@ -158,6 +158,7 @@
 #![cfg_attr(not(any(feature = "use_std", feature = "rustc-dep-of-std")), no_std)]
 // Enable lints
 #![cfg_attr(feature = "extra_traits", deny(missing_debug_implementations))]
+#![cfg_attr(feature = "extra_traits", deny(clippy::derive_hash_xor_eq))]
 #![deny(missing_copy_implementations)]
 
 #[cfg(all(not(cross_platform_docs), feature = "use_std"))]