eopl sllgen:make-rep-loop bug fix (#13831)

Attached is a simple patch that fixes a bug introduced into eopl by  Racket v5.3.4.

I initially reported the bug in June 2013, and it hasn't been fixed yet.
(see http://bugs.racket-lang.org/query/?debug=&database=default&cmd=view+audit-trail&cmd=view&pr=13831)

The fix is simple, it essentially provides the sllgen:make-rep-loop procedure from the eopl package.  This basically reverts the file to its previous state (pre Racket v5.3.4). It should be noted that in the official documentation this procedure is listed as existing.
(see http://docs.racket-lang.org/eopl/index.html?q=sllgen%3Amake#%28def._%28%28lib._eopl%2Feopl..rkt%29._sllgen~3amake-rep-loop%29%29)

It is assumed that in the restructuring process that took place between v5.3 and v5.4 the line was accidentally removed.

This patch is important for me, as I am teaching a class using Racket & EOPL soon and I'd like the students to be able to work on their homework/projects from any Racket installation, not just < 5.4.


I'm happy to discuss the changes, I've also added a very simple test that merely ensures that the identifier (sllgen:make-rep-loop) is bound in the testing environment.  The simple test fails without the change, so I think it is important to include it.


Cheers,

Andrew