Fix unused macro_use

Author: samuelburnham

src/lib.rs

@@ -51,9 +51,9 @@
 
 #![deny(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(feature = "alloc", macro_use)]
 
 #[cfg(feature = "alloc")]
-#[macro_use]
 extern crate alloc;
 
 #[cfg(any(test, feature = "arb"))]