Add FuturesUnordered into_iter method

[canndrew]

I have a FuturesUnordered that I'd like to deconstruct and get the futures out of. Currently the only way to achieve this is to wrap the futures in some kind of OptionFuture type which allows me to call FuturesUnordered::iter_mut, steal the future, then drop the FuturesUnordered before it can be polled again. This is extra boilerplate and involves adding unwraps to my code. Could FuturesUnordered support this directly?

[cramertj]

I'd be happy to accept a PR for this!