add prop:constructor-style-printer

[AlexKnauth]

This struct-type property allows structs to specify how they should by printed by constructor-style printing. I'm also submitting a pull request to pconvert-lib (racket/pconvert#6) to recognize this property and use it when converting structs into s-expressions.

It also cooperates with the pretty printer just like make-constructor-style-printer does, since it uses make-constructor-style-printer to do the printing.

> (require racket/struct)
> (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 (point 1 2))
(point 1 2)
> (parameterize ([pretty-print-columns 10])
    (pretty-print (point 3000000 4000000)))
(point
 3000000
 4000000)
> (require mzlib/pconvert)
> (print-convert (point 1 2)) ; with my other pull request to pconvert-lib
(list 'point 1 2)