Introduce awaitAll and joinAll extensions for collections for Deferreds/Jobs

joinAll is simple map { it.join() }, while awaitAll needs a slightly more involved implementation than a simple map { it.await() }. It shall actually wait on all of the deferreds from collection and crash as soon as any one of them crashes.

[fvasco]

joinAll is simple map { it.join() }

onEach should be considered to avoid collection’s copy.

[elizarov]

@raulraja We try to keep kotlinx.coroutines idiomatic with respect to Kotlin stdlib. We don’t plan to have any multiple-arity overloads. Idiomatic way to write your code is:

data class Joined<A, B, C>(a: A, b: B, c: C)

val op1: Deferred<A> = async { ... }
val op2: Deferred<B> = async { ... }
val op3: Deferred<C> = async { ... }

val results = Joined(op1.await(), op2.await(), op3.await())