Use `make-constructor-style-printer` from `racket/struct`.

[stamourv]

Moved there as of Racket 6.2.900.10.

This change is not compatible with Racket 6.2.1. You can address that by using a version exception.

Alternatively, you can add an explicit dependency to unstable-lib, which exports unstable/struct starting with Racket 6.2.900.10.