Implement common traits for SerdeSecret

Implement Debug, Eq, PartialEq and Clone for SerdeSecret if they
are implemented for the inner type.

src/serde_impl.rs

@@ -38,6 +38,7 @@ mod serialize_secret_internal {
 /// concerns serialize shouldn't be implemented for secret keys to avoid accidental leakage.
 ///
 /// Whenever this struct is used the integrity of security boundaries should be checked carefully.
+#[derive(Debug, Eq, PartialEq, Clone)]
 pub struct SerdeSecret<T>(pub T);
 
 impl<T> Deref for SerdeSecret<T> {