Add 'guix edit'.

* guix/scripts/edit.scm: New file.
* Makefile.am (MODULES): Add it.
* doc.am (SUBCOMMANDS): Add 'edit'.
* doc/guix.texi (Defining Packages): Add xref to "Invoking guix edit".
  (Invoking guix edit): New node.
* po/guix/POTFILES.in: Add it.