Added nan?, infinite?, degrees->radians, radians->degrees, exact-round, exact-floor, exact-ceiling, exact-truncate to racket/math

Altered TR's random arithmetic testing to generate single-flonums and very small flonums; fails now because of erroneous types

Fixes to sgn, sinh, cosh, and tanh:
 * preserve single-flonum-ness
 * correct zero sign (-0.0) for negative return values that are smaller than epsilon
 * correct behavior with NaN and infinite inputs

original commit: a713ca8a8b6c7aed987e80d0621484e68bc3c6f5

collects/mrlib/image-core.rkt

@@ -1132,9 +1132,6 @@ the mask bitmap and the original bitmap are all together in a single bytes!
        (values (abs rotated-width)
                (abs rotated-height)))]))
 
-(define (degrees->radians θ)
-  (* θ 2 pi (/ 360)))
-
 (define (mode-color->smoothing mode color)
   (cond
     [(and (eq? mode 'outline)