Changes

Summary

1. misc tuning and modernization;
2. discontinued obsolete 'files' in session ROOT;
3. prefer file dependencies wrt. specific theories;
4. added command 'external_file';
5. proper document (cf. 9f5bfef8bd82);
6. removed pointless dependencies: done by 'spark_open';
7. merged
8. more documentation;
9. clarified imports: prefer parent session images;
10. eliminated old-style no-document imports;
11. proper document;
12. more compact (second-order) digest for 10^2..10^3 source files, with slightly increased risk of collisions;
13. more documentation;
14. tuned;
15. sources_stamp refers to full sources; simplified data storage (again);
16. option -S for "isabelle build";
17. persistent storage of imported_sources;
18. cache sources: invoke SHA1.digest at most once; maintain imported_sources, as required for new theories;
19. tuned;
20. repaired small incident