[plt-dev] overriding constructor style printing

[Matthew Flatt]

I've added `prop:print-converter' to `mzlib/pconvert-prop' in SVN. You
can use it like this:

 (define-struct unordered (elems)
    ....
    #:property prop:print-converter (lambda (v recur)
                                      `(bag ,@(map recur (unordered-elems v))))
    ....)