Flow: withLatestFrom operator

[elizarov]

trafficstars

See #1315 for one of the use-cases. Note, that is operator can be implemented in a quite a straightforward way using stable Flow APIs. The simple implementation is given below:

fun <A, B: Any, R> Flow<A>.withLatestFrom(other: Flow<B>, transform: suspend (A, B) -> R): Flow<R> = flow {
    coroutineScope {
        val latestB = AtomicReference<B?>()
        val outerScope = this
        launch {
            try {
                other.collect { latestB.set(it) }
            } catch(e: CancellationException) {
                outerScope.cancel(e) // cancel outer scope on cancellation exception, too
            }
        }
        collect { a: A ->
            latestB.get()?.let { b -> emit(transform(a, b)) }
        }
    }
}

TODO: We need to figure out a name for this operation that is consistent with combine and xxxLatest naming convention. Even better, it should be a part of the customizable "combine family" of operations where each source flow can either react to the the incoming values or just reference the most recent value. It is also related to #1354 and #1082 which would introduce the concept of StateFlow (a flow with the most recent value).