message: Improve nextDay implementation
  Changed: Improved `nextDay` performance by roughly 50%.
diff: -import requiredArgs from '../_lib/requiredArgs/index'
  -import getDay from '../getDay'
  import addDays from '../addDays'
  -import toDate from '../toDate'
  +import getDay from '../getDay'
  import { Day } from '../types'
  -
  -const baseMap = [7, 6, 5, 4, 3, 2, 1]
  +import requiredArgs from '../_lib/requiredArgs/index'
  /**
  * @name nextDay
  @@ -31,16 +28,9 @@ const baseMap = [7, 6, 5, 4, 3, 2, 1]
  */
  export default function nextDay(date: Date | number, day: Day): Date {
  requiredArgs(2, arguments)
  - const map = genMap(day)
  - return addDays(toDate(date), map[getDay(toDate(date))])
  -}
  -function genMap(daysToMove: number): number[] {
  - if (daysToMove === 0) {
  - return baseMap
  - } else {
  - const mapStart = baseMap.slice(-daysToMove)
  - const mapEnd = baseMap.slice(0, baseMap.length - daysToMove)
  - return mapStart.concat(mapEnd)
  - }
  + let delta = day - getDay(date)
  + if (delta <= 0) delta += 7
  +
  + return addDays(date, delta)
  }
  