Make difficulty 128 bit instead of 64 bit

Based on Boolberry work by:
  jahrsg <jahr@jahr.me>
  cr.zoidberg <crypto.zoidberg@gmail.com>

tests/unit_tests/CMakeLists.txt

@@ -43,6 +43,7 @@ set(unit_tests_sources
   crypto.cpp
   decompose_amount_into_digits.cpp
   device.cpp
+  difficulty.cpp
   dns_resolver.cpp
   epee_boosted_tcp_server.cpp
   epee_levin_protocol_handler_async.cpp