removed some deprecated functions, cleaned up documentation

Instead of ... (deprecated)       | use ...
----------------------------------+---------------------------------------------------------------------------------------
extend_struct(s, field);          | s = struct_combine(s, field);
extend_struct(s, "field", value); | s = struct_combine(s, struct { field = value });
extend_struct(s, field, value);   | tmp = @Struct_Type(field); set_struct_fileds(tmp, value); s = struct_combine(s, tmp);
                                  |
s = merge_structs(s1, s2, ...);   | s = struct_combine(s1, s2, ...);
                                  |
s_ = select_struct_arrays(s, i);  | s_ = struct_filter(s, i; copy);