Core-updates merge

[Andreas Enge <andreas@enge.fr>]

Hello all,

I have just merged core-updates into master and deleted the branch!
This has been a long adventure, which became particularly intensive
after the last Guix Days in February. First and foremost many thanks to
everyone who contributed to the branch, be it by commits, discussions or
by working on the infrastructure.

Each and every package is not yet in shape; please feel free to submit
patches for your favourite packages that fail to build. In particular:
- python-yubikey-manager does not build currently; work to correct this
  is underway.
- R on powerpc does not build; this will also be corrected soon;
- aarch64 has very few substitutes; I think this is mainly due to the build
  farm catching up and not so much to packages not building, but this is
  difficult to know.
If any of these are essential to you, you may wish to wait a little bit
longer before a "guix pull", or for the time being pull to a commit just
before the merge by issuing
   guix pull --commit=472706ae2f9160833951a4e4bcc4c206e03097b0

Every end of a story is the beginning of many new ones. Several areas of
work have already been identified; I will summarise what came up on the
mailing list and who expressed interest to work on it - please feel free
to join if a topic interests you, or launch another initiative if you
want to work on a different subject!
- rust-team already has a branch that is almost ready to be built and
  merged, as a precursor to the team based workflow that we need to invent
  (Efraim Flashner).
- R on powerpc64le needs to be built by changes to valgrind and lz4
  (Simon Tournier, I).
- Many Python packages need updates, in particular with the aim of building
  python-yubikey-manager (John Kehayias, Lars-Dominik Braun, Brian Cully).
- There is still work to do to bootstrap GHC until the latest version on
  i686, and to potentially shorten the bootstrap chain (Lars-Dominik Braun).
- OCaml could be simplified by dropping version 4.07 (Julien Lepiller).
- After the mesa update is before the mesa update, and it looks like more
  features can be enabled (Kaelyn Takata, John Kehayias).
- Too much in Guix depends on too much else, which makes building things
  needlessly entangled; in particular time zone data should not be referred
  to by packages, but be loaded at runtime (Leo Famulari).

All the best in your Guix endeavours,

Andreas