rust/pv_core: Add decode_hex

It's uncommon and prone to error to silently stop decoding/parsing a
hex-string if there is an invalid character. Therefore, add a new
function `decode_hex` which fixes this behavior and use it in the code.

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

rust/pv_core/src/error.rs

@@ -68,6 +68,12 @@ pub enum Error {
     Io(#[from] std::io::Error),
     #[error(transparent)]
     ParseInt(#[from] std::num::ParseIntError),
+
+    #[error("Cannot decode hex string: Size {0} is not a multiple of two")]
+    InvHexStringSize(usize),
+
+    #[error("Cannot decode hex string")]
+    InvHexStringChar { source: std::num::ParseIntError },
 }
 
 /// Error cases for I/O operations