Fix bug: set the scope annotation in deref_operators used inside structs

Editing of error message to make it more clear.

Filling of symbol->scope annotation moved from narrow to fill_candidate_datatypes_c (became possible because narrowing of struct, array and symbolic variables is now done directly in fill_candidate_datatypes_c)

fill_candidate_datatypes_c now uses search_varinstance_decl_c instead of search_varfb_instance_type_c (moving towards deprecation of search_varfb_instance_type_c)

Do the full fill_candidate/narrow_datatype algorithm for all fields of a structured variable in the fill_candidate_datatypes_c
Allows us to remove temporary code from the search_varfb_decl_c

Fix bug with multple dereferencing (bool_var := bool_ptr_ptr^^;)

Fix bug: Add support for de-referencing pointers to struct (struct_ptr^.elem1) when used inside FB.
Note that multiple de-referencing of structs (struct_ptr_ptr^^.elem) is not supported inside FB code (this would need BIG changes to the compiler!)

Fix bug: add support for dereferencing of REF_TO to REF_TO xxx (e.g.: bool_var := ref_to_ref_to_bool^^;)

Turn support of derefencing operator '^' an option (default is not supported).

Add support for de-referencing of REF_TO datatypes (Note: dereferencing of arrays and structs not yet supported!)

Add support for REF_TO inside arrays and structs (a non-standard feature!)

Code cleanup: with the introduction of anotation_map[], generate_c_vardecl_c can now derive directly from generate_c_base_c

Fix typos in comments

Add comments, and make get_datatype_id_str_c equivalent to get_datatype_id_c

Fix calling of functions whose parameters are of an implicitly declared datatype (currently only makes sense for REF_TO datatypes, but may make sense to other datatypes too if the datatype model is changed in the future).