Make fields in Equivalence public

`Equivalence` is meant to be trivially constructible, which was the
movitation behind making it a tuple struct. However, it's still a
_struct_ and its fields are not pubilic by default. Therefore, the
type was not constructible as advertised, e.g. via `Equivalence(a, b)`.

This change makes these fields public, as they are supposed to be.

Author: neithernut

src/tester.rs

@@ -306,7 +306,7 @@ impl TestResult {
 /// }
 /// ```
 #[derive(Clone, Debug)]
-pub struct Equivalence<T>(T, T)
+pub struct Equivalence<T>(pub T, pub T)
 where
     T: Debug + PartialEq + 'static;