Warn on exhaustive exported enums

Enable clippy::exhaustive_enums in every crate so new exported enums
are either #[non_exhaustive] or explicitly allowed as a closed set.

winit-core/src/lib.rs

@@ -7,6 +7,12 @@
 //!
 //! [`winit`]: https://docs.rs/winit
 
+// Every newly exported enum should either be `#[non_exhaustive]`, or carry an `#[allow]` of
+// this lint when the set of variants can never grow.
+// `clippy::exhaustive_structs` is deliberately not enabled: event structs must stay
+// constructible by the backend crates, which `#[non_exhaustive]` would forbid.
+#![warn(clippy::exhaustive_enums)]
+
 #[macro_use]
 pub mod as_any;
 pub mod cursor;