Export url::URL

[webern]

Should we either export url::Url, e.g. as tough::Url? Or should we eliminate Url from the public interface?

I think best practice is to re-export, but I'm not sure. The reason I realized maybe we should do this is that, while working on something else, I wrote a quite test program with tough and it didn't compile because I typed url = "1" instead of url = "2". I'm sure how annoying it could be in a project to need to align Url's version with tough's use thereof (or how exact that version match needs to be).