Just use @YES and @NO internally for B_TRUE and B_FALSE.

@kiril - review

It occurred to me that B_FALSE and B_TRUE are only ever used in pointer comparisons. Because @YES and @NO are singletons, we can just use them in place of ObsConstant directly and everything should more or less just work.  Unit test pass. This can also simplify JSON serialization / deserialization code in gcstatengine.