refactor to use cffi/c2ffi to auto-generate most of it

[attila-lendvai]

hu.dwim.graphviz should be updated to use the c2ffi based machinery to auto-generate most of the FFI.

the current code uses an obsolete CFFI syntax, but changing it by hand doesn't make sense when we now have cffi/c2ffi. for an example usage of it see https://github.com/hu-dwim/hu.dwim.zlib/, or https://github.com/hu-dwim/hu.dwim.sdl/ for a more complex example.