Changes

Summary

1. merged
2. clarified Theory_Cache: prefer immutable data with Synchronized variable; clarified Export_Theory.Theory vs. Entity tables; entity_ref: proper treatment of entity kind;
3. tuned signature;
4. unused;
5. proper support of verit's return code for timeout
6. tuned whitespace;
7. updated to verit-2021.06.1-rmx, to address "Abnormal termination with exit code 14";
8. clarified signature;
9. prefer official Export.explode_name; avoid string interpolation: Isabelle/Scala is closer to Isabelle/ML than Python/Perl;
10. tuned;
11. avoid conflict with future keyword;
12. tuned messages;
13. clarified signature: more direct XML.symbol_length;
14. more direct Symbol.length: Symbol.decode is redundant, symbol counts are invariant under it;
15. tuned -- eliminate clones stemming from d28a51dd9da6;