Linking ::footnote-calls?

I don't think this is particularly clearly specified in the spec, but it would be nice if there were a way to link from a ::footnote-call site to the footnote (and potentially back). In practice these should generally be on the same page and not too hard to spot, but it's a nice usability feature, and notably, one that TeX seems to have.

I can see two paths to this:

1. There's some sort of syntax that can be used to wrap the ::footnote-call element, and we can manually make links.
2. We just automatically link footnote call sites to their footnotes (and vice versa?).

It doesn't seem as though Prince does this either, so it's not necessarily a bug in WeasyPrint, but it would be a nice feature.