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/v2/key-chain.cpp b/src/security/v2/key-chain.cpp
index c6e044c..2e5e4ad 100644
--- a/src/security/v2/key-chain.cpp
+++ b/src/security/v2/key-chain.cpp
@@ -22,8 +22,8 @@
 #include "key-chain.hpp"
 
 #include "../../util/config-file.hpp"
-#include "../../util/digest.hpp"
 #include "../../util/logger.hpp"
+#include "../../util/sha256.hpp"
 
 #include "../pib/pib-sqlite3.hpp"
 #include "../pib/pib-memory.hpp"