Support numeric types (WAS 'Allow FromStr types')

[birkenfeld]

I'd propose to allow e.g. Result<i32, darling::Error> in the struct.

I'll submit a patch if you think this is a useful idea.

[TedDriggs]

I don't think your exact suggestion is possible without specialization, but if you can make it work I'd be happy to review a PR for it.

A much simpler approach would be adding thin FromMetaItem impls for the types you care about, wrapping their FromStr impls. Those would go in [core/src/from_meta_item.rs].

With those in place, the existing blanket impl FromMetaItem<T> for Result<T> would cover the case you're describing.

[TedDriggs]

@birkenfeld I've shipped v0.2.1, which has these impls added.

[birkenfeld]

Sorry for not replying earlier. This is nice, thank you - I'll get back to you if I need FromMetaItem implemented for other types as well.