Creates an index for a datafile.
Creates an index for a datafile, to speed up loading single objects out of
it. This is mostly useful for big datafiles, which you don't want to load as
a whole. The index will store the offset of all objects inside the datafile,
and then you can load it quickly with "load_datafile_object_indexed" later.
Use destroy_datafile_index to free the memory used by it again.
Note: If the datafile uses global compression, there is no performance gain
from using an index, because seeking to the offset still requires to
uncompress the whole datafile up to that offset.
DATAFILE_INDEX *index = create_datafile_index("huge.dat");
DATAFILE *object = load_datafile_object_indexed(index, 1234);
A pointer value which you can pass to load_datafile_object_indexed.