security: Reorganizing source code to prepare for support of two version of NDN certificates

This commit also removes unused ndn_digestSha256 function and deprecates
crypto::sha256 in favor of crypto::computeSha256Digest in util/crypto.hpp.

Change-Id: I24ee50ff073a96b868633bdf2cfade412d3605f3
Refs: #3098
diff --git a/tests/unit-tests/security/key.t.cpp b/tests/unit-tests/security/key.t.cpp
index 27db02c..681c43c 100644
--- a/tests/unit-tests/security/key.t.cpp
+++ b/tests/unit-tests/security/key.t.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2015 Regents of the University of California.
+ * Copyright (c) 2013-2016 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -74,7 +74,7 @@
   BOOST_REQUIRE_NO_THROW(key11.setDefaultCertificate(id1Key1Cert1));
   BOOST_REQUIRE_NO_THROW(key11.getDefaultCertificate());
 
-  const IdentityCertificate& defaultCert = key11.getDefaultCertificate();
+  const v1::IdentityCertificate& defaultCert = key11.getDefaultCertificate();
   BOOST_CHECK_EQUAL_COLLECTIONS(defaultCert.wireEncode().wire(),
                                 defaultCert.wireEncode().wire() + defaultCert.wireEncode().size(),
                                 id1Key1Cert1.wireEncode().wire(),