Use the new feature flag for procedural macros

Author: migerh

src/lib.rs

@@ -49,7 +49,7 @@
 //! }
 //! ```
 
-#![feature(proc_macro, wasm_custom_section, wasm_import_module)]
+#![feature(use_extern_macros, wasm_custom_section, wasm_import_module)]
 
 #[macro_use]
 extern crate cfg_if;