chore: fix macro usage order to prevent potential compilation errors (#1372)

Author: famouswizard

examples/u256/src/main.rs

@@ -2,11 +2,11 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![allow(clippy::needless_range_loop)]
 
-openvm::entry!(main);
 use core::array;
-
 use openvm_bigint_guest::U256;
 
+openvm::entry!(main);
+
 const N: usize = 16;
 type Matrix = [[U256; N]; N];