[bug#42738] [Work in progress] gnu: emacs: update to 27.1

[Jack Hill]

Hi Guix,

Now that there is a release candidate for Emacs 27.1, I started working on 
updating our Emacs packages. For the most part, it went smoothly, but 
there are some other packages that broke after updating Emacs. Of the 
packages reported by `guix refresh -l emacs`, the following had issues:

emacs-picpocket - picpocket uses ImageMagic, but Emacs no longer defaults 
to using ImageMagic [0]. This may not be a package that we want to support 
with Emacs >=27

[0] 
http://git.savannah.gnu.org/cgit/emacs.git/tree/etc/NEWS?h=emacs-27.1-rc1&id=27877e7bcfa37b2c97a3dde170f870d4729ff807#n96

bigloo - Currently fails to build [1], and the Emacs upgrade did not 
change that.

[1] https://issues.guix.gnu.org/42726

hop - depends on bigloo

emacs-haskell-mode - Tests newly fail with Emacs 27. Issue reported 
upstream [2]

[2] https://github.com/haskell/haskell-mode/issues/1714

emacs-dante - depends on emacs-haskell-mode

emacs-clojure-mode - Documentation tests newly fail wtih Emacs 27. Issue 
reported upstream [3]

[3] https://github.com/clojure-emacs/clojure-mode/issues/568

emacs-ample-regexps - This package builds from an arbitrary commit, 
and updating that fixed issues with Emacs 27 [4].

[4] https://issues.guix.gnu.org/42737

emacs-doom-themes - This package builds from an arbitrary commit, and 
updating that fixed issues with Emacs 27 [5].

[5] https://issues.guix.gnu.org/42736

emacs-treemacs - Tests newly fail with Emacs 27. Issue reported upstream [6].

[6] https://github.com/Alexander-Miller/treemacs/issues/707

emacs-lsp-java - depends on treemacs

emacs-treemacs-extra - depends on treemacs

emacs-psc-ide - Byte compilation fails with Emacs 27. Issue reported 
upstream [7].

[7] https://github.com/purescript-emacs/psc-ide-emacs/issues/193

emacs-treepi - Byte compilation failure. There is an existing upstram 
issue [8].

[8] https://github.com/volrath/treepy.el/issues/8

emacs-github-review - depends on emacs-treepi

notmuch - Tests time out.

emacs-elisp-ref - Tests fail. Issue reported upstream [9].

[9] https://github.com/Wilfred/elisp-refs/issues/29

emacs-helpful - depends on emacs-elisp-ref

emacs-racer - Tests fail. Issue reported upstream [10].

[10] https://github.com/racer-rust/emacs-racer/issues/136

emacs-md4rd - Byte compilation fails. Issue reported upstream [11].

[11] https://github.com/ahungry/md4rd/issues/28

Having said that, I will send the patch that I have for updating Emacs. 
This borrows heavily from the current definition of emacs-next. One thing 
I don't understand is why I had to add texinfo to the native inputs of 
emacs-minimal. Perhaps that can be removed with a configure option. That 
said, `guix size` reports a decrease from 205.3 MiB to 182.9 MiB for 
emacs-minimal (and emacs decreased from 1209.8 MiB to 859.7 MiB).

I've also left emacs-next alone for now. We'll probably want to update it 
as well at some point.

Best,
Jack