recognize and use prop:constructor-style-printer when converting structs

[AlexKnauth]

This changes print-convert to work with my other pull request to racket (racket/racket#1337) so that it can recognize the new prop:constructor-style-printer property and use it when converting structs that have it.

> (require racket/struct mzlib/pconvert)
> (struct point (x y) ; this struct is opaque and wouldn't print like this otherwise
    #:property prop:constructor-style-printer
    (list
     (lambda (p) 'point)
     (lambda (p) (list (point-x p) (point-y p)))))
> (print-convert (point 1 2))
(list 'point 1 2)
> (constructor-style-printing #true)
> (print-convert (point (list 1 2 3) (list 4 5 6)))
(list 'point (list 'list 1 2 3) (list 'list 4 5 6))