Nextafter 根据x和y的值，返回某一方向最接近x的值

特殊情况: Nextafter(x, x) = x Nextafter(NaN, y) = NaN Nextafter(x, NaN) = NaN <iframe style="border:1px solid" src="https://wide.b3log.org/playground/954a603f4febf60ccb6e8525f2387790.go?embed=true" width="99%" height="600"></iframe>