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.