Add Generic instance for Maybe

[LiamGoodacre]

No description provided.

[paf31]

Seems fine to me, thanks. Curious what you need this for though, since typically I'd expect Maybe to be used as an Argument or Field, but not as Generic itself.

[LiamGoodacre]

My motivation is that as we do more interesting things with interpreting generics-rep structures (like with generating optics, encodings, etc) it seems appropriate to be able to apply them not only to our own data structures but also existing ones like Maybe (which I think is the only data structure this library depends on?)

[paf31]

Thanks!