Create typed structs for PIN-protected and admin metadata (#223)

MgmKey::set_protected and YubiKey::set_pin_last_changed both contained
bugs resulting from the conversion of C pointer logic (incorrect buffer
management). The new Metadata struct holds its own buffer, avoiding the
problem.

Also adds a protected management key integration test.

tests/integration.rs

@@ -107,6 +107,35 @@ fn test_verify_pin() {
     assert!(yubikey.verify_pin(b"123456").is_ok());
 }
 
+//
+// Management key support
+//
+
+#[cfg(feature = "untested")]
+#[test]
+#[ignore]
+fn test_protected_mgmkey() {
+    let mut yubikey = YUBIKEY.lock().unwrap();
+
+    assert!(yubikey.verify_pin(b"123456").is_ok());
+    assert!(yubikey.authenticate(MgmKey::default()).is_ok());
+
+    // Set a protected management key.
+    assert!(MgmKey::generate()
+        .unwrap()
+        .set_protected(&mut yubikey)
+        .is_ok());
+    let protected = MgmKey::get_protected(&mut yubikey).unwrap();
+    assert!(yubikey.authenticate(MgmKey::default()).is_err());
+    assert!(yubikey.authenticate(protected.clone()).is_ok());
+
+    // Set back to the default management key.
+    // TODO: This does not clear the previous key from the protected metadata.
+    assert!(MgmKey::default().set(&mut yubikey, None).is_ok());
+    assert!(yubikey.authenticate(protected).is_err());
+    assert!(yubikey.authenticate(MgmKey::default()).is_ok());
+}
+
 //
 // Certificate support
 //