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/data.t.cpp b/tests/unit-tests/data.t.cpp
index e38cd98..48f4f82 100644
--- a/tests/unit-tests/data.t.cpp
+++ b/tests/unit-tests/data.t.cpp
@@ -27,7 +27,7 @@
 #include "security/transform/step-source.hpp"
 #include "security/transform/stream-sink.hpp"
 #include "security/verification-helpers.hpp"
-#include "util/digest.hpp"
+#include "util/sha256.hpp"
 
 #include "boost-test.hpp"
 #include "identity-management-fixture.hpp"