Break module cycle involving (guix store) and (guix ui).

Break module cycle involving (guix store) and (guix ui).

Before, there was a cycle along the lines of:

  (guix store) -> (guix nar) -> (guix ui) -> (guix store)

This caused problems, as discussed at:

  http://lists.gnu.org/archive/html/guix-devel/2014-10/msg00109.html

This patch removes cycles in the (guix ...) modules.

* guix/nar.scm (&nar-error, &nar-read-error, dump, write-contents,
  read-contents, %archive-version-1, write-file, restore-file): Move to...
* guix/serialization.scm: ... here.
* guix/store.scm: Remove dependency on (guix nar).
* guix/scripts/hash.scm, guix/scripts/offload.scm,
  guix/scripts/substitute-binary.scm, tests/nar.scm, tests/store.scm,
  tests/substitute-binary.scm: Adjust accordingly.