Support proc-macro2/nightly

[kdy1]

Fixes #23.

::darling style paths only works with Span::call_site().

Note that tests fails with proc_macro::__internal::with_sess() called before set_parse_sess()! if proc- macro2/nightly is enabled because proc_macro's apis cannot be called from non-proc-macro code.

[TedDriggs]

::darling style paths only works with Span::call_site().

I'm not fully understanding this; do you mind expanding on it a bit? Thanks for submitting this PR; going to test it locally then will approve and ship an updated crate if all goes well.

[kdy1]

extern crate from call_site (including user-written ones) does not work for def_site because macro hygiene doesn't want spans from multiple places to be mixed.