First half of set 2

examples/set2_9/main.rs

@@ -0,0 +1,13 @@
+use cryptopals::friendly::pad_pkcs7;
+
+fn main() {
+    assert_eq!(
+        pad_pkcs7(b"YELLOW SUBMARINE".to_vec(), 20),
+        b"YELLOW SUBMARINE\x04\x04\x04\x04"
+    );
+    assert_eq!(
+        pad_pkcs7(b"".to_vec(), 8),
+        b"\x08\x08\x08\x08\x08\x08\x08\x08"
+    );
+    println!("pass!")
+}