Adding compatibility with HoTT for 8.19

[MevenBertrand]

trafficstars

There was a bit of porting involved, but not much.

Ideally, it would be nice to get this into opam, I guess it is only a matter of doing a release here + a PR on the coq/opam side. I can try to take care of the PR for opam if you don't have the cycles @mattam82, (that way, I'll learn a new skill), but in any case you'll have to do a release first, I guess?

Also, I only did the change for 8.19 because that's the version I was interested in, but I can also port this to other versions if there is interest (although I guess it can't go into main because coq-hott is not following Coq's master?).

[MevenBertrand]

Sorry to ping you @mattam82, but I'm still somewhat blocked because of this, any chance of getting HoTT back on track soon-ish?