Callbacks added with asyncfutures.addCallback are invoked in reverse order

[endragor]

I expected that addCallback respects the order in which I add callbacks, but turns out it calls them in reverse order - each new callback is prepended to the linked list, instead of being appended. I think it makes more sense to append them, especially given the name (in stdlib "add" always appends), and there are use cases where ordering is important.