Merge branch 'master' into core-updates

hash: Add 'open-sha256-input-port', for Guile > 2.0.9.

* guix/hash.scm (open-sha256-input-port): New procedure.
* tests/hash.scm (supports-unbuffered-cbip?): New procedure.
  ("open-sha256-input-port, empty", "open-sha256-input-port, hello",
  "open-sha256-input-port, hello, one two",
  "open-sha256-input-port, hello, read from wrapped port"): New tests.

hash: Add `open-sha256-port'.

* guix/hash.scm (GCRY_MD_SHA256): New macro.
  (sha256): Use it.
  (open-sha256-md, md-write, md-read, md-close, open-sha256-port,
  port-sha256): New procedures.
* tests/hash.scm: New file.
* Makefile.am (SCM_TESTS): Add it.

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).