factor::miller_rabbin::Result: Mark as #[must_use]

Ignoring a value of that type is a bug: they are only produced by
`miller_rabbin::test`, which is a pure, but expensive, function.

As such, an ignored value is always either a mistake, or an easy
optimisation opportunity (just remove the useless call to `test`).

src/uu/factor/src/miller_rabin.rs

@@ -25,6 +25,7 @@ impl Basis for Montgomery<u32> {
 }
 
 #[derive(Eq, PartialEq)]
+#[must_use = "Ignoring the output of a primality test."]
 pub(crate) enum Result {
     Prime,
     Pseudoprime,