fails:Float#next_float returns a float the smallest possible step greater than the receiver
fails:Float#next_float steps directly between MAX and INFINITY
fails:Float#next_float steps directly between 1.0 and 1.0 + EPSILON
fails:Float#next_float steps directly between -1.0 and -1.0 + EPSILON/2
fails:Float#next_float reverses the effect of prev_float
fails:Float#next_float returns negative zero when stepping upward from just below zero
fails:Float#next_float returns NAN if NAN was the receiver
