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/security/v1/public-key.cpp b/src/security/v1/public-key.cpp
index 7ada249..9e245e9 100644
--- a/src/security/v1/public-key.cpp
+++ b/src/security/v1/public-key.cpp
@@ -25,7 +25,7 @@
 #include "public-key.hpp"
 
 #include "../../encoding/oid.hpp"
-#include "../../util/digest.hpp"
+#include "../../util/sha256.hpp"
 #include "cryptopp.hpp"
 
 namespace ndn {