* Add a new `CtrlC` struct which will represent a stream of SIGINT
signals on Unix or the CTRL_C event on Windows
* `CtrlC` implements `Stream<Output = ()>` rather than `IoSteam` as
previously
Migrate to std::futures and the futures 0.3 preview and use async/await
where possible
**Breaking change:** the IoFuture and IoStream definitions used to refer
to Box<dyn Future> and Box<dyn Stream>, but now they are defined as
Pin<...> versions which are technically breaking.
No other breaking or functional changes have been made
Ivan Petkov