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

Conflicts:
	gnu-system.am

guix hash: Add '--recursive'.

* guix/scripts/hash.scm (show-help): Add --recursive.
  (%options): Likewise.
  (guix-hash)[file-hash]: New procedure.  Honor --recursive.
  Use it.
* guix/nar.scm (write-file): Add missing field to the &nar-error
  condition raised upon unsupported file type; change its message to be
  more descriptive.
* tests/guix-hash.sh: Add tests with -r.
* doc/guix.texi (Invoking guix hash): Document --recursive.

guix hash: Don't load the whole file in memory.

* guix/scripts/hash.scm (guix-hash)[eof->null]: Remove.
  (guix-hash): Use 'port-sha256' to compute the hash instead of
  'get-bytevector-all' and co.

Move `sha256' to (guix hash).

* guix/utils.scm (sha256): Move to...
* guix/hash.scm: ... here.  New file.
* Makefile.am (MODULES): Add it.
* guix/derivations.scm, guix/scripts/download.scm,
  guix/scripts/hash.scm, guix/scripts/refresh.scm, tests/base32.scm,
  tests/derivations.scm, tests/store.scm: Use (guix hash).

tests: Add `guix hash' test.

* guix/scripts/hash.scm (guix-hash)[eof->null]: New procedure.
  Use it to convert the EOF object to the empty bytevector.
* tests/guix-hash.sh: New file.
* Makefile.am (SH_TESTS): Add it.

ui: Add `args-fold*' and use it.

* guix/ui.scm (args-fold*): New procedure.
* guix/scripts/build.scm, guix/scripts/download.scm,
  guix/scripts/gc.scm, guix/scripts/hash.scm, guix/scripts/import.scm,
  guix/scripts/package.scm, guix/scripts/pull.scm,
  guix/scripts/refresh.scm: Use `args-fold*' instead of `args-fold'.

Merge branch 'master' into core-updates

Conflicts:
	Makefile.am
	gnu/packages/autotools.scm
	gnu/packages/guile.scm
	gnu/packages/python.scm
	gnu/packages/shishi.scm
	guix/gnu-maintenance.scm
	guix/scripts/build.scm
	guix/scripts/gc.scm
	guix/scripts/package.scm
	guix/scripts/substitute-binary.scm
	guix/ui.scm
	nix/nix-daemon/guix-daemon.cc
	test-env.in
	tests/nar.scm
	tests/store.scm

Add 'guix hash'.

* guix/scripts/hash.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Add it.
* doc/guix.texi (Invoking guix hash): New node.
  (Defining Packages): Add a cross-reference to the 'Invoking guix
  hash' node.