get index of first structure type
returns: -1 if no structure type is defined
index of first structure type.
Each structure type has an index and ID.
INDEX determines position of structure definition
in the list of structure definitions. Index 1
is listed first, after index 2 and so on.
The index of a structure type can be changed any
time, leading to movement of the structure definition
in the list of structure definitions.
ID uniquely denotes a structure type. A structure
gets a unique ID at the creation time and this ID
can't be changed. Even when the structure type gets
deleted, its ID won't be resued in the future.
long get_first_struc_idx();