Changes

Summary

1. NEWS;
2. updated documentation;
3. more conventional theory syntax for ML bootstrap, with 'ML_file' instead of 'use'; avoid slowdown of Resources.loaded_files due to command name 'use' in Pure base syntax;
4. unused (see caaa2fc4040d);
5. simplified default print_depth: context is usually available, in contrast to 0d295e339f52;
6. clarified bootstrap of @{make_string} -- avoid query on ML environment;
7. Pure attribute setup is back to Pure/Isar/attrib.ML, where it can be editing continuously (see also 7eb0c04e4c40);
8. prefer regular context update, to allow continuous editing of Pure;
9. clarified editor mode;
10. treat ROOT.ML as theory with header "theory ML_Root imports ML_Bootstrap begin";
11. more robust bootstrap;
12. virtual thread data via context, for proper support of Context.>> etc;
13. unused;
14. tuned signature;
15. clarified bootstrap;
16. clarified modules; tuned signature;
17. proper return code;
18. clarified ML bootstrap environment;
19. simplified bootstrap: critical structures remain accessible in ML_Root context;
20. more uniform cleanup (via ML_Process in Scala);
21. clarified bootstrap;
22. clarified ML bootstrap;