Fix grading and feedback so that extra entries are not needed in `data['correct_answers']`

[glherman]

The truth table grading and rendering currently depends on creating extra entries in data['correct_answers']. Best practice would be to have only the entry that exactly matches the name of the element (i.e., data['correct_answers']['ans'] if answers-name="ans"). There should not be extra entries like data['correct_answers'][['ans_0_0']. Update grading and rendering to remove need for the extra entries in data['correct_answers'].