for await-latest ?

Hi, we need to be able to iterate over async sequence such that if the iteration does some async stuff & upstream happens to emit in the middle of it, the current iteration is cancelled & next one is subscribed (i.e. it doesn't wait)

```
for item in items.latest {
try await sleep(long time)
}
```

this would mirror kotlin's `collectLatest (also there's flatMapLatest, transformLatest` which are very powerful & necessary

Start by reviewing the repository's existing AsyncSequence APIs and the issue's Kotlin collectLatest comparison; no implementation files or tests are named. Define the cancellation and resubscription behavior for items.latest, then add coverage showing that an in-progress iteration is cancelled when a newer upstream item arrives.

Written by the indexing model from the issue text.