A new wip-emacs branch

[Leo Prikler]

Hello Guix,

as at least some of you are hopefully aware, the way Emacs interacts
with Guix packaging is unsatisfactory in a few key ways.  In
particular, each major version upgrade completely breaks Emacs both
running and not yet running until environment variables are updated
[1].  Also, there are instances of packages breaking each other by
installing to common subdirectories [2].

I have opened up a new wip-emacs branch to address these issues.  It
consists of the patches I wrote in the past few days, that are still
awaiting review.  (As it is now April 1st, people who only consume the
mailing lists by the archives will soon have forgotten about them
otherwise).  While alternative patches exist, particular the ones
written up by Maxim, I believe mine to be the "correct" ones, as they
only cause rebuilds to Emacs and its dependants, thereby making them
applicable to staging rather than core-updates.  (In the past we also
had Emacs patches pushed directly to master, since Emacs packages are
fairly cheap to build; I want to avoid this here until we can be
certain up to some level of reasonable doubt, that they do not cause
any issues in the affected packages.)

I have so far tested the patches by running my own Emacs manifest in a
pure environment, which has not yet led to me declaring .emacs
bankruptcy.  I would strongly encourage other users of Emacs,
particularly those, that have large numbers of Emacs packages in their
profiles, to try out the wip-emacs branch and report to me any issues
with it/directly push patches to wip-emacs if they're trivial.

I don't plan to keep this branch alive for too lang.  In one or two
weeks time, depending on activity, I will submit its frozen version to
review once more.  In this frozen version, I will also sign off any
commits from others, that I've already reviewed myself.

Regards,
Leo

[1] http://issues.guix.gnu.org/47458
[2] http://issues.guix.gnu.org/45316