Replace `VecDeque` with `stream::from_iter` in examples

[yoshuawuyts]

Now that rust-lang/futures-rs#1930 has landed, we're waiting on #415 to be merged. Once it's merged we should replace all uses of VecDeque with stream::from_iter so we're setup for the futures@0.3.0 release.

This should be a fairly mechanical change, and easy to pick up!

[k-nasa]

Will I do this with #415 ?

[yoshuawuyts]

@k-nasa I think we should merge that first, and create a follow-up PR to it that ports over the examples. That way we keep moving forward without having a single massive PR that does both things at once. Would you be willing to write the follow-up PR?