Changes

Summary

1. more portable options;
2. build rsync from sources, to avoid divergence of protocols on various platforms;
3. more informative errors;
4. clarified options;
5. tuned messages;
6. provide Isabelle tool wrapper;
7. more robust errors: proceed updating database; clarified options; clarified progress;
8. tuned;
9. tuned output;
10. clarified signature;
11. clarified modules;
12. tuned comments (amending 1951f6470792);
13. tuned;
14. discontinue somewhat pointless is_single, which also depends on details of internal data representation;
15. more compact data: approx. 0.85 .. 1.10 of plain list size; fewer comparisons for Leaf2 / Leaf3: observe order;
16. slightly more compact data;