emacs: Add development utils.

* emacs/guix-guile.el (guix-guile-current-module): New function.
* emacs/guix-devel.el: New file.
* emacs.am (ELFILES): Add it.
* doc/emacs.texi (Emacs Development): New node.
  (Emacs Interface): Add it.
* doc/contributing.texi (The Perfect Setup): Mention it.
* doc/guix.texi (Top): Add it.
* emacs/guix-init.el: Add 'guix-devel-activate-mode-maybe' to
  'scheme-mode-hook'.