Hello,

The following adds support for byte compiling the generated autoload
files as well as the site-lisp directory of Emacs (the one containing
the site-start.el and guix-emacs.el files).

The performance gain is roughly 30%, as expected for byte compiled
Elisp.  The following command was used for benchmarking:

--8<---------------cut here---------------start------------->8---
./pre-inst-env guix environment -v 10 -m ~/100-emacs-packages-manifest.scm -- emacs -Q --batch --eval "(progn (require 'guix-emacs) (message \"%s\" (benchmark-run-compiled 10 (guix-emacs-autoload-packages))))"
--8<---------------cut here---------------end--------------->8---

It loads the autoloads of ~100 Emacs packages 10 times (~1000 autoloads
files).  The manifest file 100-emacs-packages-manifest.scm is attached.