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/src/security/secured-bag.cpp b/src/security/secured-bag.cpp
index 1780c3a..66fad02 100644
--- a/src/security/secured-bag.cpp
+++ b/src/security/secured-bag.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2014 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).
  *
@@ -41,7 +41,7 @@
   this->wireDecode(wire);
 }
 
-SecuredBag::SecuredBag(const IdentityCertificate& cert, ConstBufferPtr key)
+SecuredBag::SecuredBag(const v1::IdentityCertificate& cert, ConstBufferPtr key)
   : m_cert(cert)
   , m_key(key)
   , m_wire(tlv::security::IdentityPackage)