`cond` should be refactored to `when` or `unless` when applicable

[jackfirth]

#lang resyntax/test

test: "cond should be refactorable to unless when equivalent"
--------------------
#lang racket
(define (f)
  (cond
    [c (void)]
    [else
     (define x 42)
     (* x 2)]))
--------------------
--------------------
#lang racket
(define (f)
  (unless c
    (define x 42)
    (* x 2)))
--------------------

Saw this in racket/drracket#698.