Struct std::JoinAll

generic<F: Future, T> struct JoinAll {
    []F futs;
    []T outs;
    unsigned long long done;
}

Polls every future in 'futs' until all are Ready, storing future i's output in outs[i] (futures::future::join_all, without the heap). Output: how many (= futs.len). At most 64 futures.