add experimental async/await support. (tokio-rs#582)

This patch adds experimental async/await support to Tokio. It does this
by adding feature flags to existing libs only where necessary in order
to add nightly specific code (mostly `Unpin` implementations). It then
provides a new crate: `tokio-async-await` which is a shim layer on top
of `tokio`.

The `tokio-async-await` crate is expected to look exactly like `tokio`
does, but with async / await support. This strategy reduces the amount
of cfg guarding in the main libraries.

This patch also adds `tokio-channel`, which is copied from futures-rs
0.1 and adds the necessary `Unpin` implementations. In general, futures
0.1 is mostly unmaintained, so it will make sense for Tokio to take over
maintainership of key components regardless of async / await support.

add experimental async/await support. (tokio-rs#582)

This patch adds experimental async/await support to Tokio. It does this
by adding feature flags to existing libs only where necessary in order
to add nightly specific code (mostly `Unpin` implementations). It then
provides a new crate: `tokio-async-await` which is a shim layer on top
of `tokio`.

The `tokio-async-await` crate is expected to look exactly like `tokio`
does, but with async / await support. This strategy reduces the amount
of cfg guarding in the main libraries.

This patch also adds `tokio-channel`, which is copied from futures-rs
0.1 and adds the necessary `Unpin` implementations. In general, futures
0.1 is mostly unmaintained, so it will make sense for Tokio to take over
maintainership of key components regardless of async / await support.

Bump version to v0.1.8 (tokio-rs#566)

This also bumps a number of sub crates:

* tokio-executor (0.1.3)
* tokio-io (0.1.8)
* tokio-reactor (0.1.4)
* tokio-threadpool (0.1.6)
* tokio-timer (0.2.6)
* tokio-udp (0.1.2)

Bump version to v0.1.8 (tokio-rs#566)

This also bumps a number of sub crates:

* tokio-executor (0.1.3)
* tokio-io (0.1.8)
* tokio-reactor (0.1.4)
* tokio-threadpool (0.1.6)
* tokio-timer (0.2.6)
* tokio-udp (0.1.2)

Bump version to v0.1.8 (tokio-rs#566)

This also bumps a number of sub crates:

* tokio-executor (0.1.3)
* tokio-io (0.1.8)
* tokio-reactor (0.1.4)
* tokio-threadpool (0.1.6)
* tokio-timer (0.2.6)
* tokio-udp (0.1.2)

Bump version to v0.1.8 (tokio-rs#566)

This also bumps a number of sub crates:

* tokio-executor (0.1.3)
* tokio-io (0.1.8)
* tokio-reactor (0.1.4)
* tokio-threadpool (0.1.6)
* tokio-timer (0.2.6)
* tokio-udp (0.1.2)

Bump version to v0.1.8 (tokio-rs#566)

This also bumps a number of sub crates:

* tokio-executor (0.1.3)
* tokio-io (0.1.8)
* tokio-reactor (0.1.4)
* tokio-threadpool (0.1.6)
* tokio-timer (0.2.6)
* tokio-udp (0.1.2)

Bump version to v0.1.8 (tokio-rs#566)

This also bumps a number of sub crates:

* tokio-executor (0.1.3)
* tokio-io (0.1.8)
* tokio-reactor (0.1.4)
* tokio-threadpool (0.1.6)
* tokio-timer (0.2.6)
* tokio-udp (0.1.2)

Bump tokio-uds version to v0.2.1 (tokio-rs#552)

Fixes tokio-rs#551