make the drracket/tool-lib library
exports and drracket:tool^ be in sync

This doesn't quite do the whole job
because something seems to get names
confused when structs and signatures
team up. This is a small example that, I
believe, demonstrates the problem:

(require racket/unit)

(struct p:s ())
(define p:struct:s struct:p:s)

(define-signature s1^
  ((struct s ())))

(define-signature s2^
  ((open (prefix p: s1^))))

(define-values/invoke-unit
  (unit (import s2^) (export))
  (import s2^)
  (export))
avoid invalidating the line numbers when doing things like drawing arrows in check syntax