add bulletproofs to the build, with basic unit tests

Based on Java code from Sarang Noether

tests/unit_tests/CMakeLists.txt

@@ -34,6 +34,7 @@ set(unit_tests_sources
   blockchain_db.cpp
   block_queue.cpp
   block_reward.cpp
+  bulletproofs.cpp
   canonical_amounts.cpp
   chacha8.cpp
   checkpoints.cpp