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

[casey]

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.