add_struc

define a new structure type
     index    - Not used anymore
     name     - name of the new structure type.
     is_union - 0: structure
                1: union

returns: -1 if can't define structure type because of bad structure name:
            the name is ill-formed or is already used in the program.
         otherwise returns ID of the new structure type

long add_struc(long index, string name, long is_union);