Tests for non-unit variant enum rotations

[nullstalgia]

Ties into #1.

While experimenting with this macro, especially in a UI context, I found a repeated desire to have more than just unit variants cyclable in my enums.

Example:

#[derive(EnumRotate)]
enum Enum {
    MainMenu,
    Manual { page: u8 },
    Credits,
}

I'll often have Enums like this one, where there's either a flow or a collection of states where the persistence of data across those states isn't of importance.

So using calls to Default::default() for each field seemed like a reasonable compromise to me, although I would understand if you would prefer breaking this out into another trait/macro pair.

I didn't edit any documentation or try to tackle every Clippy suggestion I saw, as I wanted to get your feedback first. :)

Thanks for the useful macro!