emacs: Use general functions instead of generated ones.

* emacs/guix-base.el: Add and use general functions instead of
  specialized functions generated by 'guix-define-buffer-type' macro.
  (guix-buffer-type, guix-entry-type): New variables.
  (guix-set-vars): Add new variables.
  (guix-get-show-entries, guix-show-entries, guix-set-buffer)
  (guix-history-call, guix-make-history-item)
  (guix-get-params-for-receiving, guix-revert-buffer)
  (guix-redisplay-buffer): New functions.
  (guix-define-buffer-type): Do not generate specialized functions.
* emacs/guix-info.el (guix-package-info-insert-full-names): Use
  'guix-get-show-entries'.
  (guix-generation-info-insert-number): Likewise.
* emacs/guix-list.el (guix-list-describe): New function.
  (guix-list-define-entry-type): Do not generate specialized one.
  (guix-generation-list-show-packages): Use 'guix-get-show-entries'.
* emacs/guix.el (guix-show-generations-function): Remove.
  (guix-get-show-packages): Use new functions.
  (guix-get-show-generations): Likewise.