Add predicates to work with struct pointers

[aarroyoc]

Some libraries like SDL require you to pass a struct that they fill. This was nearly impossible to do right now with our FFI interface, so three more predicates have been added.

SDL_Event event;
SDL_PollEvents(&event)

can be expressed as:

alloc_struct_ptr(sdl_event, Ptr),
ffi:'SDL_PollEvent'(Ptr, _),
unfold_struct_ptr(sdl_event, Ptr, [sdl_event, 512, ...]),
dealloc_struct_ptr(sdl_event, Ptr)