KT-24658 Add downUntil function

[trevorhackman]

I thought https://youtrack.jetbrains.com/issue/KT-24658 would be an easy one for me to do and contribute, first time contributor here.

downUntil feels like a simple but missing stdlib function in my opinion. As said in the issue 6 years ago, "Kotlin has .. for upward closed ranges, downTo for downward closed ranges, and until for upward half-open ranges.", but no downUntil for downward half-open ranges.

[JakeWharton]

These will also need intrinsics so that they don't allocate for simple loops, but that can be tracked/added separately.

[trevorhackman]

These will also need intrinsics so that they don't allocate for simple loops, but that can be tracked/added separately.

Nice, I wasn't aware of that. Would it be pretty similar to the existing DownToHandler.kt or UntilHandler.kt? I could have a go at adding a DownUntilHandler.kt too unless a separate ticket & PR would be better.

[JakeWharton]

I don't think it matters all that much. I would probably wait, to ensure these are acceptable APIs first.