tokio: provide future / stream / sink helpers

[carllerche]

Currently, the goal is to update master to use std::future. There currently are no future helpers provided by std. However, there are helpers provided by futures-rs and other locations.

tokio must decide how it will depend on external crates for both private and public usage as well as what helpers tokio will provide itself.

Fix TODO

This list tracks places in the code base that had temporary code applied to get things working and need a second pass.

• tokio-threadpool/tests/threadpool.rs
• tokio-timer/tests/hammer.rs
• tokio-timer/tests/timeout.rs

Refs: rust-lang/futures-rs#1701, rust-lang/futures-rs#1700

[carllerche]

Switch to futures-sink: #1244

[rubdos]

This sounds like it's "decided" to use futures-preview then? A Stream trait would be very useful, since even the examples cannot be cleanly written without one. Even if you wrote #1242 (async fn accept()) to avoid importing Stream, iterating over items in Framed also becomes cumbersome! (Found out while rewriting examples)

[carllerche]

As of now, Tokio will add a public dependency on the Stream / Sink traits. Internals, tests, and examples can use the helpers for those traits provided in futures-util.