WIP gnu: Add (GNUnet-)groupchat.

[Tanguy Le Carrour]

Dear Guix,

The WIP in the subject might be a bit misleading, because I haven't
actually started to work on the packaging! This being said…

I've recently decided to spend a little time working/helping on GNUnet.

The first step was to have the latest version running on my machine.
I've sent some patches [1][] and I've been happily running a node
(0.12.2) for few days now.

[1]: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=39673

The second step is to use some of the tools on top of GNUnet!

The `re:claimID` [2][] extension works but somehow the demo site [3][]
doesn't. I opened an issue upstream and moved to `groupchat` [4][].

[2]: https://reclaimid.gitlab.io
[3]: https://demo.reclaim-identity.io/login
[4]: https://gnunet.org/en/use.html#groupchat

I started by updating nim to 1.0.6 (patch accepted) and played a bit with
the code. `groupchat` seems to be a little software with only one
dependency… but I'm kinda stuck! The only thing I've figured out so far is
that I cannot rely on `nimble` (the package manager `groupchat` uses)!

Any help/advice would really be highly appreciated!

-- 
Tanguy