[racket] List abbreviations in Beginning Student

Hi Barry,

> I noticed that beginning students are able to use list abbreviation constructors. It doesn't return the list in abbreviated form, however.
>
> (list 1 2 3)
> returns
> (cons 1 (cons 2 (cons 3 empty)))
>
> Is that correct?

I've noticed some overlaps of functionality between the teaching
languages myself too. I remember using this feature when I first
encountered the list abbreviations though, since for a while I wasn't
confident what a nested (list ... (list ...)) might look like in the
cons form.

As a side note, a lot of people have trouble with the arrangements
exercise in section 12, which is a little cleaner to work with using
list abbreviations, so thats another possible reason for including
this functionality in the beginning student language.

Horace.