It would be nice if strings formed with quotations marks knew their position
(filename, line number, column number).  Same for strings returned by "get".  And
"lines" and "substring" should preserve the positions.  That would make the code
in SimpleDoc much simpler, and it would mean that error messages from "value"
would generally be much more informative.

This could be implemented by a new Type of String called LocatedString.
