Changes

Summary

1. enforce rebuild of Isabelle/ML to update build databases;
2. prefer relative src_path (if possible) -- in contrast to 9ce0aa145d21:
3. proper treatment of unicode_symbols;
4. tuned signature: avoid alias that is unclear wrt. lazy state and Symbol.encode/decode status;
5. removed unused operation: unclear wrt. Symbol.encode/decode status;
6. tuned signature: more uniform operations;
7. tuned comments;
8. unused;
9. more uniform operations; plain file_name instead of blob.src_path.implode_short;
10. restrict to proper_session_theories;
11. proper build parameters (amending d858e6f15da3);
12. treat update_options as part of Sessions.Info meta_digest, for proper re-build of updated sessions;
13. more command-line options;