Converters from LocalDate and LocalDateTime to NSDate

[dkhalanskyjb]

We received a few requests to add converters from LocalDate and LocalDateTime to NSDate. Given that NSDate is essentially the same as Instant, they could look something like this:

fun LocalDate.toNSDateAtStartOfDayIn(timeZone: TimeZone) =
  atStartOfDayIn(timeZone).toNSDate()

fun LocalDateTime.toNSDate(timeZone: TimeZone) =
  toInstant(timeZone).toNSDate()

They seem quite short, but if they are tricky to get right, maybe we could introduce them to the library.