Implement challenge 9 pkcs 7 padding.

src/bytes.rs

@@ -67,6 +67,14 @@ impl Bytes {
         r
     }
 
+    pub fn pad_pkcs7(&mut self, block_size: usize) -> () {
+        let Bytes(v) = self;
+        let padding_value = (block_size - v.len() % block_size) as u8;
+        for _ in 0..padding_value {
+            v.push(padding_value);
+        }
+    }
+
     pub fn xor(Bytes(a): &Bytes, Bytes(b): &Bytes) -> Bytes {
         Bytes(
             Iterator::zip(a.iter(), b.iter())