reflect bsl, etc. printer options for external use

collects/htdp/asl/lang/reader.rkt

@@ -1,4 +1,5 @@
 #lang s-exp htdp/bsl/reader
 lang/htdp-intermediate-lambda
 '(abbreviate-cons-as-list
-  read-accept-quasiquote)
+  read-accept-quasiquote
+  show-sharing)

collects/htdp/bsl/reader.rkt

@@ -14,6 +14,7 @@
    #:info (make-info options)
    #:language-info (make-language-info options)
 
+   (provide options)
    (define options opts)))
 
 (define (wrap-reader read-proc options)

collects/htdp/bsl/runtime.rkt

@@ -28,6 +28,7 @@
   ;; Set pretty-print options:
   (pretty-print-show-inexactness #t)
   (pretty-print-exact-as-decimal #t)
+  (show-sharing (memq 'show-sharing options))
 
   ;; Set print handlers to use print-convert and pretty-write:
   (current-print