Edited the task for lab2.
Minor corrections, edits and bug fixes.

Transfered show_menu() function to car_adt2_test.c
Checked scanf() return value where needed.
Cleared the input buffer after each scanf().
Added some more assert()-ions.
Removed the empty line from the end of file.
Removed the empty line from the end of the file.
Small fixes and refactoring.

Changed the name of fflush_scanf() to fflush_stdin();
Solved redeclared identifier filled;
Removed the empty line from the end.
Removed the function show_menu() which is not part of the ADT.
Massive refactoring.

1. Added some TODOs;
2. Removed an include;
3. Made all test functions static;
4. Added more test functions;
5. Separated main in smaller functions;
6. Transferred show_menu() function here;
7. Added a bunch of functions in order to make each function small, testable,
   with maximum 3 levels of identation.