#6 Use `char *` instead of `FILE *` for graph decoder input

The [decode function prototype](https://git.knowledgetx.com/scossu/lsup_rdf/src/master/include/codec.h#L151) in the codec interface uses `FILE *` as an input. It should use `char *` for handling in-memory strings more easily. Optionally, an additional wrapper function can be written to work directly file handles.