cs: faster number->string on flonums

pkgs/racket-test-core/tests/racket/number.rktl

@@ -179,6 +179,8 @@
 (test "+nan.0" number->string +nan.0)
 (test "+nan.0" number->string +nan.0)
 
+(test "1589935744527.254" number->string 1589935744527.254)
+
 #reader "maybe-single.rkt"
 (begin
   (test (if has-single-flonum? "+inf.f" "+inf.0") number->string +inf.f)

racket/src/cs/rumble/number.ss

@@ -393,6 +393,13 @@
       result)]
    [else
     (cond
+     [(and (eq? radix 10)
+           (number? n))
+      ;; `number->string` goes through `format` to get to an implementation
+      ;; like this, so take a shortcut:
+      (let ([op (#%open-output-string)])
+        (#%display n op)
+        (#%get-output-string op))]
      [(eq? radix 16)
       ;; Host generates uppercase letters, Racket generates lowercase
       (string-downcase (#2%number->string n radix))]