New “ungrafting” branch

[Ludovic Courtès]

Hi Guix!

Following discussions on IRC, I’ve created a new ‘ungrafting’ branch
that does nothing but ungraft things.

The rationale is that grafts incur additional overhead when installing
things (the time to create those grafts), so it’s good to clean them up
once in a while.  Ungrafting in a dedicated branch means we know the
branch is “safe”, unlike more exploratory branches like ‘staging’ and
‘core-updates’.

The plan is to start building it later today, and to hopefully be done
in a week or so.

Thoughts?

Let’s ungraft!

Ludo’.