Serialize announce URLs as `Url` instead of `String`

Announce URLs are serialized as `String`, instead of `Url`. I'm not sure why I do this, since `Url` supports serde with the `serde` feature enabled.

I'm inclined to start serializing URLs as `Url`, instead of `String`, since it is slightly less error prone. However, this would cause some torrents with invalid announce URLs to stop deserializing correctly. I think this is rare, but I'm still on the fence about whether or not this downside is worth it.