Working on union signature. It seems that libffi doesn't have any special support for unions, but with some customisations of existing support for struct signatures, unions also works for function which makes and returns some union type. This need to be more tested. Alse fixed bug related to signature_types_to_list, and function still need some improvements, but I'll work on that. Next will be to try make union signature when we need to pass union as function argument.