* Fix crate path in `Cargo.toml` of examples
* Add `edition2018` to examples in the documentation to make it compiled
on Rust 2018
* Fix an example in the documentation
- Rewrite noop_waker with items from the new API and replaces
LocalWaker with Waker.
- Bump the minimum required version for `tokio-async-await` to
1.34.0-nightly.
- `Unpin` was added to std prelude.
- Add `cargo check` to .travis.yml
Fixes: #908
Error::cause is deprecated in Rust 1.33, but this allows Error::cause
until the minimum supported version of tokio is Rust 1.30.
When the minimum support version of tokio reaches Rust 1.30,
replace Error::cause with Error::source.
Fixes: #817