Changes

Summary

1. merged
2. isabelle_process executable no longer supports writable heap images;
3. more careful cleanup;
4. obsolete;
5. tuned;
6. redundant -- already part of Session.finish;
7. proper exit as in Scala version (in contrast to a45ba78abcc1);
8. save heap more directly;
9. clarified modules;
10. clarified ML heap operations;