Changes

Summary

1. merged
2. Simplified some proofs
3. merged
4. tuned signature;
5. tuned signature: avoid warnings in IntelliJ IDEA;
6. tuned signature;
7. tuned signature;
8. tuned signature;
9. clarified state: more explicit type as plain value, which is also easier to sync with external db;
10. tuned signature;
11. tuned signature;
12. clarified signature: support meaningful subclasses for Build.Engine implementations;
13. support alternative build engines, via system option "build_engine";
14. misc tuning and clarification; support SSH.System;
15. proper test, following Platform.is_linux;
16. clarified signature;
17. clarified signature;
18. merged
19. Simplified some more proofs
20. merged
21. Replacing z powr of_int i by z powi i and adding new material from the AFP