Changes

Summary

1. eliminated clones;
2. trim blanks -- more thoroughly than in update_cartouches (for single-line comments);
3. more operations (as in ML);
4. more specific tool "isabelle update_comments", for uniformity with formal comments in embedded languages;
5. clarified Token.is_text (cf. Parse.text in ML);
6. more operations;
7. support for completion;
8. allow LaTeX source as formal comment;
9. clarified signature;
10. clarified SML (no formal comments) vs. Isabelle/ML (arbitrary comments);
11. clarified signature;
12. clarified modules: uniform notion of formal comments;