allow for/fold #:results to reference sequence identifiers

Someone in the Racket Slack channel requested being able to reference the sequencing identifiers in the #:results clause of a for/fold. I think this would be a straightforward addition; I plan on looking into it in the near future.

I think the below two examples illustrate how this feature should behave:

#lang racket

(require rackunit)

;; the #:result clause can reference identifiers
;; bound by the sequencing clauses
(check-equal? (for/fold ([sum 0]
                         #:result (list sum n))
                        ([n (in-range 3)])
                (+ sum n))
              (list 3 2))

;; the accumulators bindings shadow the bindings
;; of identifiers bound by sequencing clauses
(check-equal? (for/fold ([n 0]
                         #:result n)
                        ([n (in-range 3)])
                (number->string n))
              "2")