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.