Convert from proc_macro2::Span to proc_macro::Span without panic

[TedDriggs]

Right now, it looks like proc_macro2::Span::unwrap is the only way to get a proc_macro::Span, but that will panic if it's called outside a proc macro. Could there be a TryFrom/TryInto impl, or a method that returns Option<proc_macro::Span>?

[dtolnay]

I think this is supported well enough by proc_macro::is_available().then(|| span2.unwrap()).