pki: Add 'signature-case' macro.

* guix/pki.scm (%signature-status): New procedure.
  (signature-case): New macro.
* tests/pki.scm (%secret-key, %alternate-secret-key): New variables.
  ("signature-case valid-signature", "signature-case invalid-signature",
  "signature-case hash-mismatch", "signature-case unauthorized-key",
  "signature-case corrupt-signature"): New tests.

Merge branch 'master' into core-updates

Add (guix pki).

* guix/pki.scm, tests/pki.scm: New files.
* Makefile.am (MODULES): Add 'guix/pki.scm'.
  (SCM_TESTS): Add 'tests/pki.scm'.