Portable pickling of anonymous tuples

[daveraja]

Pull request #75 adds pickling of anonymous tuples. But it uses sys._getframe() which is an cpython specific implementation function. So to support a more portable approach need to look at alternatives. The idea is to transform the anonymous tuple into a normal tuple during the pickling process (in the Predicate.__setstate__() function )and during unpickling (in the Predicate.__getstate__() function), turn the tuple back into the appropriate anonymous tuple.

Note: I think this idea should work for anonymous tuples that are part of an otherwise pickleable predicate sub-class definition. However, it would not solve the problem for other dynamically generated predicate sub-classes, such as using the simple_predicate() function.