feat: finish p18, implment aes ctr mode

problems/p15/src/main.rs

@@ -1,5 +1,6 @@
 use anyhow::{Result, anyhow};
 
+#[allow(dead_code)]
 fn pkcs7_unpadding(input: &[u8]) -> Result<Vec<u8>> {
     if input.is_empty() {
         return Err(anyhow!("Input cannot be empty"));