define a new structure type
index - index of new structure type
If another structure has the specified index,
then index of that structure and all other
structures will be increment freeing the specified
index. If index is == -1, then the biggest index
number will be used.
See get_first_struc_idx() for the explanation of
structure indices and IDs.
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);