Add 'guix challenge'.

* guix/scripts/challenge.scm, tests/challenge.scm: New files.
* Makefile.am (MODULES): Add the former.
  (SCM_TESTS): Add the latter.
* doc.am (SUBCOMMANDS): Add 'challenge'.
* doc/guix.texi (Substitutes): Add xref to 'guix challenge'.
  (Invoking guix challenge): New node.
* doc/contributing.texi (Submitting Patches): Add note about using 'guix
  challenge'.
* po/guix/POTFILES.in: Add guix/scripts/challenge.scm.