rust: Fix cargo clippy and cargo doc findings

Fix the new `cargo clippy` and `cargo doc` findings that were triggered
with the recent policy addition.

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

rust/utils/src/hexslice.rs

@@ -30,7 +30,7 @@ where
     }
 }
 
-impl<'a> Serialize for HexSlice<'a> {
+impl Serialize for HexSlice<'_> {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: serde::Serializer,