Added doctest for LETREC compiler; added NULL special form.

Spent over an hour to work out two things:

1. Missed some brackets, so changed

    return [NIL] + compile_app(args, n, [LD] + index(fcn, n) + [AP] + c)

    to

    return [NIL] + compile_app(args, n, [LD] + [index(fcn, n)] + [AP] + c)

    and;

2. Example of LETREC on p. 164 of Kogge [1991] had a missing closing
   paren, and I wasted some time trying something of the form

   (LETREC (f) (LAMBDA ... ))

   instead of

   (LETREC (f) ((LAMBDA ... ))

   which was my own fault.