In an effort to reach API stability, the `tokio` crate is shedding its
_public_ dependencies on crates that are either a) do not provide a
stable (1.0+) release with longevity guarantees or b) match the `tokio`
release cadence. Of course, implmementing `std` traits fits the
requirements.

Instead of publically dependending on `futures` from `tokio`, the
`tokio-util` crate provides the necessary bridge to go from Tokio types
to `futures` traits (primarily `Stream`).

Instead of implementing `Stream` for iteration, types that are logically
streams provide `async fn next_*` functions to obtain the next value.
Avoiding the `next()` function name will allow for forwards
compatibility once the `Stream` trait is provided by `std`.

Additionally, some misc cleanup is also done:

- `tokio::io::io` -> `tokio::io::util`.
- `delay` -> `delay_until`.
- `Timeout::new` -> `timeout(...)`.
- `signal::ctrl_c()` returns a future instead of a stream.
- `{tcp,unix}::Incoming` is removed (due to lack of `Stream` trait).
- `time::Throttle` is removed (due to lack of `Stream` trait).