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/data.cpp b/src/data.cpp
index 724335e..054c975 100644
--- a/src/data.cpp
+++ b/src/data.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).
  *
@@ -184,7 +184,7 @@
                                   "(e.g., not signed)"));
     }
     m_fullName = m_name;
-    m_fullName.appendImplicitSha256Digest(crypto::sha256(m_wire.wire(), m_wire.size()));
+    m_fullName.appendImplicitSha256Digest(crypto::computeSha256Digest(m_wire.wire(), m_wire.size()));
   }
 
   return m_fullName;