Added _curies support.

Curies get attached to `LinkCollection` since that's where they appear and matter, and then used to expand links. Open to refactoring this in some different way.

Expanding a curie is as simple as replacing `{rel}` in the curie's href by the URL, unless I am missing something. `URITemplate` would encode the URL if you said `expand(rel: href)`, so I got stuck with a `gsub`. I could be wrong and there's a better way to do this.