wrapping of structs with private members

[Laeeth]

It looks like LuaD tries to wrap private members and then fails here:
conversions/structs.d-mixin-38(38,1)

I'll try to fix it when time.

also, would be nice to be able to specify manual function to wrap specified D types for the occasional cases when that is helpful (apologies if I am being dense and haven't seen it). in pyd you can specify a delegate for D->python and python->D:

• This allows the user to define a function for returning a D type to Python. The dg may be either a function pointer or a delegate. The argument to the function pointer is of the type to convert. The return type of the function pointer can be any convertible type.
• This allows the user to define a function for converting a Python object to a D type. The dg may be either a function pointer or a delegate. The argument to the function pointer can be any convertible type. The return type of the function pointer is the type to convert.