Don't derive From by default for variants without fields

[JelteF]

There's not really a reason to derive from for () for variants without fields. It's somewhat unexpected and results in the problems described here in #105. Let's remove this in the next breaking release, probably 1.0.

[Robbepop]

Could you also please make a warning if a crate has some spurious #[from(ignores)] that it would no longer need with version 1.0. I currently am having many of those #[from(ignore]) annotations in my crates and it would be a blessing to have some indicators by the macro itself if they are no longer useful.