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.