Solve the rest of the compiler warnings.

src/error/error.rs

@@ -18,7 +18,7 @@ impl<I> ParseError<I> for CustomError<I> {
         CustomError::Nom(input, kind)
     }
 
-    fn append(_input: I, _kind: ErrorKind, mut other: Self) -> Self {
+    fn append(_input: I, _kind: ErrorKind, /*mut*/ other: Self) -> Self {
         // Doesn't do append like VerboseError
         other
     }