Fix segfault in util.df and improve performance by struct preallocation (see #46)

FFI bindings were generated using [gcc-lua-cdel](http://colberg.org/gcc-lua-cdecl/). We could integrate this into our building system to avoid further segfault problems as the one discussed in #46. However, we would need to agree into a convention for ffi.cdefs, for example, we could have a `ffi/name.c` file specifying the fuctions, structs, types, variables and consts to be exported to `ffi/name.lua`. The generated lua file would contain only a `ffi.cdef`, and it would be require'd by modules which use these ffi declarations.