rust: Add a Sealed trait

Some traits exposed by the kernel crate may not be intended to be
implemented by downstream modules. Add a Sealed trait to allow avoiding
this using the sealed trait pattern.

Signed-off-by: Asahi Lina <lina@asahilina.net>

Author: hoshinolina

rust/kernel/lib.rs

@@ -57,6 +57,11 @@ pub use uapi;
 #[doc(hidden)]
 pub use build_error::build_error;
 
+pub(crate) mod private {
+    #[allow(unreachable_pub)]
+    pub trait Sealed {}
+}
+
 /// Prefix to appear before log messages printed from within the `kernel` crate.
 const __LOG_PREFIX: &[u8] = b"rust_kernel\0";