Changes

Summary

1. NEWS;
2. provide POLYSTATSDIR to keep $HOME/.polyml clean (requires Poly/ML 52881757b127, otherwise ignored);
3. adapted to 7b318273a4aa;
4. cache props;
5. clarified names; removed remains of old protocol messages;
6. more thorough protocol_handlers.exit, like file_formats.stop_session;
7. tuned names;
8. temporary workaround for 100% CPU usage in OS.Process.sleep;
9. ML statistics via external process: allows monitoring RTS while ML program sleeps;
10. clarified catch-all handler --- avoid confusion of Interrupt vs. Exn.Interrupt in varying ML contexts;
11. avoid failure of "isabelle build -o skip_proofs";