major cleanup;
removed several obsolete functions;
moved file stuff to Thy/file.ML;
How to build TFL and run the Unify example.
1. Invoke the current version of Isabelle-HOL.
2. use "sys.sml";
3. cd "examples/Subst"; or 3. cd "examples";
4. use "ROOT.ML"; 4. use"test.sml";