feat(wpcarro/emacs): Package al, list, set, struct

Originally I set-out to package `al.el`, but as I started traversing the
dependencies, I needed to package increasingly more packages. I refactored some
of these to prune their dependencies to slay this hydra before it turned into a
never-ending project. I have mixed feelings about this.

I also introduced `ert` and unit tests into my Elisp packaging, so it'll be nice
to have build-time tests that run when Emacs updates land in depot.

Change-Id: I2756dc60888b80255a495e08ae61bd547e6b3db2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5998
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/emacs/default.nix

@@ -25,6 +25,10 @@ let
 
   wpcarrosEmacs = emacsWithPackages (epkgs:
     (with wpcarro.emacs.pkgs; [
+      al
+      list
+      set
+      struct
       zle
     ]) ++