Hello Guix!

This patch set strengthens our Emacs build system by enforcing strict
byte compilation of every Guix package using the `emacs-build-system'.

Before, errors during byte compilation would not signal a failure of the
build.

It also adds a custom check phase to make it easy to run any kind of
program or script that might be used as a test runner (commonly
ert-runner, sometimes make, etc.).

I've validated that our Emacs packages (at least those whose name start by
'emacs-') can be successfully built with those changes (240 were built
locally). Most of the patches below were necessary fixes to our Emacs
packages so that they could be successfully byte compiled.

Happy April Fools' Day ;)

Maxim