enforce rebuild of Isabelle/ML to update build databases;

prefer relative src_path (if possible) -- in contrast to 9ce0aa145d21:

proper treatment of unicode_symbols;

tuned signature: avoid alias that is unclear wrt. lazy state and Symbol.encode/decode status;

removed unused operation: unclear wrt. Symbol.encode/decode status;

tuned signature: more uniform operations;

tuned comments;

unused;

more uniform operations; plain file_name instead of blob.src_path.implode_short;

restrict to proper_session_theories;

proper build parameters (amending d858e6f15da3);

treat update_options as part of Sessions.Info meta_digest, for proper re-build of updated sessions;

more command-line options;

tuned options --- avoid confusion with "isabelle build -b";

tuned signature;

isabelle update -u path_cartouches;

merged

updated documentation;

more options; tuned messages;

tuned message;

isabelle update no longer uses PIDE dump, but regular session build database: more scalable; misc tuning and clarification;

more robust;

more operations; more robust;

proper Node.init_blobs, not just edits (amending ca872f20cf5b);

tuned signature;

tuned signature;

clarified session sources: theory and blobs are read from database, instead of physical file-system;

tuned;

clarified signature: more operations;

clarified signature: more operations;