Replace non-greedy regexp with a char-complement to improve matching performance.

Measurements show a runtime improvement of about 10% for string-heavy
JSON documents.  http://lists.racket-lang.org/users/archive/2013-January/055953.html

collects/json/main.rkt

@@ -107,7 +107,7 @@
   (define (read-string)
     (let loop ([l* '()])
       ;; note: use a string regexp to extract utf-8-able text
-      (define m (cdr (or (regexp-try-match #rx"^(.*?)(\"|\\\\(.))" i)
+      (define m (cdr (or (regexp-try-match #rx"^([^\"\\]*)(\"|\\\\(.))" i)
                          (err "unterminated string"))))
       (define l (if ((bytes-length (car m)) . > . 0) (cons (car m) l*) l*))
       (define esc (caddr m))