Rename or remove futures_unordered module in 0.2

[aturon]

The fact that we have a function and module both named futures_unordered is confusing both to people and to rustdoc.

[cramertj]

I think it's fine to remove the function now that we have the FromIterator impl which does the same thing. I think the only reason the module exists is so that there's a place for IterMut, but this could probably be renamed to FuturesUnorderedIterMut or something and then reexported from the stream module.

[alexcrichton]

I'd also be fine removing iteration as it didn't solve the original problem anyway