Add combinators for Future?

[adamchalmers]

Hi! Firstly I just want to say I love this page and use it all the time. Thanks for making it, it's really helpful for teaching new coworkers their way around Rust.

I was curious: would you be OK if I, or someone else, added combinators for Future (e.g. map/then) and maybe Stream?

Upsides:

• now that futures are in stdlib, they're much more common in Rust

Downsides:

• FutureExt and Streams aren't in stdlib yet, so they might change, causing churn for this repo.

[jethrogb]

Yes, this sounds great!