Moving Fibers to Codeberg

Hello!

Following Guix, the Shepherd, and now Guile, I’d like to propose to move
Fibers from <https://github.com/wingo/fibers/> to Codeberg.  I haven’t
heard from Aleix yet but I know that Andy is in favor.

The import from GitHub is pretty much lossless: issues and pull requests
are preserved, with the same numbers.  All we need to do is to click the
button on Codeberg, update README on GitHub, and then mark the GitHub
repo as archived.  I’m happy to take care of this part.

An open question is continuous integration: we could use Codeberg’s
Woodpecker, or we could hook into Cuirass at ci.guix or elsewhere
(though the version of Cuirass with pull request integration is not
deployed yet on ci.guix).

Thoughts?

Ludo’.