Update after proc_macro stabilization

The feature `proc_macro` was recently stabilized [1]. This has two effects:
- The `proc_macro` feature gate is replaced by finer grained feature gates
- Using this crate now doesn't need `#![feature(proc_macro)]` but
  `#![feature(use_extern_macros)]`

[1]: rust-lang/rust#52081

Author: LukasKalbertodt

compile_test/src/main.rs

@@ -2,7 +2,7 @@
 Try running `cargo expand` on this crate to see the output of `#[auto_impl]`.
 */
 
-#![feature(proc_macro)]
+#![feature(use_extern_macros)]
 
 extern crate auto_impl;
 

examples/greet_closure.rs

@@ -1,4 +1,4 @@
-#![feature(proc_macro)]
+#![feature(use_extern_macros)]
 
 extern crate auto_impl;
 

examples/refs.rs

@@ -1,4 +1,4 @@
-#![feature(proc_macro)]
+#![feature(use_extern_macros)]
 
 extern crate auto_impl;
 

src/lib.rs

@@ -4,7 +4,8 @@
 #![feature(crate_in_paths)]
 #![feature(extern_prelude)]
 #![feature(in_band_lifetimes)]
-#![feature(proc_macro)]
+#![feature(proc_macro_span)]
+#![feature(proc_macro_diagnostic)]
 
 
 extern crate proc_macro;