Update to fine-grained proc_macro feature gates

Author: dtolnay

src/lib.rs

@@ -44,7 +44,7 @@
 
 // Proc-macro2 types in rustdoc of other crates get linked to here.
 #![doc(html_root_url = "https://docs.rs/proc-macro2/0.4.7")]
-#![cfg_attr(feature = "nightly", feature(proc_macro))]
+#![cfg_attr(feature = "nightly", feature(proc_macro_raw_ident, proc_macro_span))]
 
 #[cfg(feature = "proc-macro")]
 extern crate proc_macro;