exercise util: generate more useful process_X_case defaults

[coriolinus]

When generating process_X_case functions, the current implementation causes the compiler to complain about unused variables. We should, instead of generating comments describing what it might look like, just generate the example directly. I.e. using the abbreviate case:

fn process_abbreviate_case<I, O>(input: I, expected: O) {
    assert_eq!(
        student_abbreviate_func(input), // TODO: what is the actual name of student_abbreviate_func?
        expected
    );
}