Enabling proc-macro2/nightly breaks this crate

[kdy1]

error[E0433]: failed to resolve. Could not find `darling` in `{{root}}`
 --> macros\ast_node\src\spanned.rs:3:17
  |
3 | #[derive(Debug, FromMetaItem)]
  |                 ^^^^^^^^^^^^ Could not find `darling` in `{{root}}`

This is a known issue. dtolnay/proc-macro2#67

quote! use Span::def_site() by default, which is internally Span::call_site() on stable.