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/src/data.cpp b/src/data.cpp
index 649eb7a..e982700 100644
--- a/src/data.cpp
+++ b/src/data.cpp
@@ -21,7 +21,7 @@
 
 #include "data.hpp"
 #include "encoding/block-helpers.hpp"
-#include "util/digest.hpp"
+#include "util/sha256.hpp"
 
 namespace ndn {