Make index? faster.

collects/typed-scheme/types/numeric-predicates.rkt

@@ -1,6 +1,6 @@
 #lang racket/base
 
-(require unstable/function)
+(require unstable/function racket/unsafe/ops)
 
 (provide index? exact-rational?)
 
@@ -9,6 +9,7 @@
 ;; we assume indexes are 2 bits shorter than fixnums
 ;; We're generating a reference to fixnum? rather than calling it, so
 ;; we're safe from fixnum size issues on different platforms.
-(define (index? x) (and (fixnum? x) (>= x 0) (fixnum? (* x 4))))
+(define (index? x)
+  (and (fixnum? x) (unsafe-fx>= x 0) (fixnum? (unsafe-fx* x 4))))
 
 (define exact-rational? (conjoin rational? exact?))