Auto implement Same for types that impl Eq.

[andrewjstone]

If we want to retain backwards compatibility for f32 and f64 we'll need to put this behavior behind a feature that doesn't allow diffus to work with f32 and f64. This is a good tradeoff to make so that nobody has to implement Same manually.

Alternatively, if we wanted to go full hog and break backwards compat, we could just remove Same and force types to implement Eq.

CC @sunshowers