Implement challenge 9 pkcs 7 padding.

src/set2.rs

@@ -0,0 +1,11 @@
+use crate::bytes::Bytes;
+
+pub fn challenge9() {
+    let mut bytes = Bytes::from_utf8("YELLOW SUBMARINE");
+    bytes.pad_pkcs7(20);
+    println!("[okay] Challenge 9: {:?}", bytes.to_utf8());
+}
+
+pub fn challenge10() {
+    println!("[TBD] Challenge 10: TBD");
+}