Fix according to latest clippy

Author: xxuejie

tools/ckb-sphincs-tools/src/utils.rs

@@ -70,7 +70,7 @@ fn char_to_u8(c: char) -> u8 {
 pub fn str_to_bytes(input: &str) -> Vec<u8> {
     let off = if input.contains("0x") { 2 } else { 0 };
     let input = input.as_bytes();
-    assert!(input.len() % 2 == 0);
+    assert!(input.len().is_multiple_of(2));
     let mut r = vec![0; (input.len() - off) / 2];
 
     for i in 0..r.len() {