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/util/io.t.cpp b/tests/unit-tests/util/io.t.cpp
index f8ea2ae..b789020 100644
--- a/tests/unit-tests/util/io.t.cpp
+++ b/tests/unit-tests/util/io.t.cpp
@@ -248,11 +248,11 @@
   identity.appendVersion();
   BOOST_REQUIRE(addIdentity(identity, RsaKeyParams()));
   Name certName = m_keyChain.getDefaultCertificateNameForIdentity(identity);
-  shared_ptr<IdentityCertificate> idCert;
+  shared_ptr<security::v1::IdentityCertificate> idCert;
   BOOST_REQUIRE_NO_THROW(idCert = m_keyChain.getCertificate(certName));
 
   io::save(*idCert, filename);
-  shared_ptr<IdentityCertificate> readCert = io::load<IdentityCertificate>(filename);
+  shared_ptr<security::v1::IdentityCertificate> readCert = io::load<security::v1::IdentityCertificate>(filename);
 
   BOOST_CHECK(readCert != nullptr);
   BOOST_CHECK_EQUAL(idCert->getName(), readCert->getName());