emacs: Add and use alist accessors.

* emacs/guix-utils.el (guix-define-alist-accessor): New macro.
  (guix-assq-value, guix-assoc-value): New functions.
  (guix-get-key-val): Remove.
* emacs/guix-base.el: Replace 'guix-get-key-val' with 'guix-assq-value'
  everywhere.
* emacs/guix-info.el: Likewise.
* emacs/guix-list.el: Likewise.
* emacs/guix-messages.el: Likewise.