util: rename digest.hpp to sha256.hpp

The new file name matches the Sha256 class included within.

This commit also deletes deprecated crypto.hpp.

refs #3940

Change-Id: Ib9bbcd0ba087b812bb0886ceaea116689d4633f5
diff --git a/tests/unit-tests/util/digest.t.cpp b/tests/unit-tests/util/sha256.t.cpp
similarity index 99%
rename from tests/unit-tests/util/digest.t.cpp
rename to tests/unit-tests/util/sha256.t.cpp
index d8614c1..6e42201 100644
--- a/tests/unit-tests/util/digest.t.cpp
+++ b/tests/unit-tests/util/sha256.t.cpp
@@ -19,7 +19,7 @@
  * See AUTHORS.md for complete list of ndn-cxx authors and contributors.
  */
 
-#include "util/digest.hpp"
+#include "util/sha256.hpp"
 #include "util/string-helper.hpp"
 #include "encoding/endian.hpp"