uORB: add bitset for faster orb_exists check and remove uORB::Subscription lazy subscribe hack/optimization

- add PX4 bitset and atomic_bitset with testing
 - add uORB::Subscription constructor to take ORB_ID enum
 - move orb test messages into msg/

platforms/posix/cmake/sitl_tests.cmake

@@ -4,8 +4,10 @@
 
 # TODO: find a way to keep this in sync with tests_main
 set(tests
+	atomic_bitset
 	autodeclination
 	bezier
+	bitset
 	bson
 	commander
 	controllib