An Atom URI. There are lots of rules on which combinations of links are permitted in one feed. See RFC4287 Sec 4.1.1 for the gory details.
val mk_link : ?rel:[ `self | `alternate ]->?typ:string->Uri.t->link
mk_link ~rel ~typ uri builds a link. rel defaults to `self, and typ represents the optional MIME type (e.g. text/html). The uri should usually be a fully qualified URI.