Add new eo.rs module for the Eo object.

- The main Eo object is now located in the eo module.
- Improve get_c_args into to_c_args.
- Re-factor functions using to_c_args.
- Implement EinaInlist::new method (wrapper for *_EinaInlist).
- Port ~str to StrBuf in modules and tests.
- Use CString instead of the from_c_str function.