Remove futures_[un]ordered functions

[taiki-e]

Closes #1477

[Nemo157]

Personally I'd go for @cramertj's proposal in the linked issue, remove the futures_unordered function and use FuturesUnordered::from_iter instead. Then the iterators can be namespaced away since they're going to be very rarely used types.

Probably good to wait and see if @cramertj's opinion has changed since then though.

[cramertj]

I don't have a strong opinion either way. @Nemo157 feel free to review this and merge if it matches your expectations.

[Nemo157]

Thanks, this looks great.

[olivia-fl]

I don't know if I'm missing something, but what should people be doing instead of futures_ordered now?

[Nemo157]

@Benjamin-L you can use the FromIterator implementation of FuturesOrdered. Either by using some_futures.into_iter().collect::<FuturesOrdered>() or use std::iter::FromIterator; FuturesOrdered::from_iter(some_futures); (I really wish that FromIterator made it into the prelude so that that could be the goto recommendation).

[Arnavion]

Needs to be mentioned in the alpha.14 changelog since it's a breaking change. (Took me a bunch of git-blame-ing to discover this.)