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.

Merge branch 'master' into core-updates

store: Add #:timeout build option.

* guix/serialization.scm (write-string-pairs): New procedure.
* guix/store.scm (write-arg): Add 'string-pairs' case.
  (set-build-options): Add 'timeout' keyword parameter.  Honor it.
* tests/derivations.scm ("build-expression->derivation and timeout"):
  New test.

Add (guix nar) and (guix serialization).

* guix/store.scm (write-int, read-int, write-long-long, read-long-long,
  write-padding, write-string, read-string, read-latin1-string,
  write-string-list, read-string-list, write-store-path,
  read-store-path, write-store-path-list, read-store-path-list): Move to
  serialization.scm.
  (write-contents, write-file): Move to nar.scm.
* guix/nar.scm, guix/serialization.scm: New files.
* Makefile.am (MODULES): Add them.

Add (guix nar) and (guix serialization).

* guix/store.scm (write-int, read-int, write-long-long, read-long-long,
  write-padding, write-string, read-string, read-latin1-string,
  write-string-list, read-string-list, write-store-path,
  read-store-path, write-store-path-list, read-store-path-list): Move to
  serialization.scm.
  (write-contents, write-file): Move to nar.scm.
* guix/nar.scm, guix/serialization.scm: New files.
* Makefile.am (MODULES): Add them.