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/validator.cpp b/src/security/validator.cpp
index 4862d76..bb8ffda 100644
--- a/src/security/validator.cpp
+++ b/src/security/validator.cpp
@@ -20,8 +20,8 @@
  */
 
 #include "validator.hpp"
-#include "../util/digest.hpp"
 #include "../lp/tags.hpp"
+#include "../util/sha256.hpp"
 
 #include "v1/cryptopp.hpp"