emacs: Add API for 'guix-entry'.

* emacs/guix-info.el: Use new entry procedures.
* emacs/guix-list.el: Likewise.
* emacs/guix-base.el: Likewise.
  (guix-get-entry-by-id): Move and rename to ...
* emacs/guix-entry.el (guix-entry-by-id): ...this.  New file.
  (guix-entry-value, guix-entry-id, guix-entries-by-ids)
  (guix-replace-entry): New procedures.
* emacs.am (ELFILES): Add new file.