Changes

Summary

1. merged
2. proper object-logic constraint (amending dd2914250ca7);
3. reconcile object-logic constraint vs. mixfix constraint;
4. more explicit support for object-logic constraint;
5. more language markup;
6. more accurate mixfix type constraints;
7. tuned;
8. tuned message;
9. more explicit type; tuned;
10. relevant check_mixfix happens further at the bottom, to avoid duplicate reports via Specification.prepare;
11. avoid duplicate reports;
12. tuned messages -- position is usually missing here;
13. more PIDE markup; tuned;
14. clarified modules;
15. clarified errors: more positions;
16. clarified simple mixfix;
17. tuned;
18. more operations;
19. updated dependencies;
20. updated to Navigator 2.6;