Allow to abort & get back values from `JoinAll`

[piegamesde]

I have a (relatively small) list of futures that need to be polled. But at any time, I may want to abort the operation and get the futures or the value they resolved back. I see that JoinAll uses a list of MaybeDone internally, which is exactly what I'm looking for. So I propose adding an into_inner method (open for bike shedding) that allows me to get the futures back.

[taiki-e]

Probably we can add methods to return iterators that return the internal future as an item.