Changes

Summary

1. misc tuning and modernization;
2. tuned whitespace;
3. added isabelle jedit -R; errors in session_info/session_content are ignored and deferred to later checks of Build.build;
4. misc tuning and modernization;
5. more standard notation (like infix);
6. dummy fork to produce ML_statistics even in sequential mode (e.g. for heap size);
7. expose stderr, e.g. Multithreading.tracing;
8. test parallel proof terms in this small session (somewhat slow for bigger applications);
9. tuned messages -- more symbols;
10. tuned;
11. more permissive syntax; more PIDE markup;