The `hyper::rt::Executor` trait allows defining custom executors to be
used with hyper's `Client` and `Server`.
Closes#1944
BREAKING CHANGE: Any type passed to the `executor` builder methods must
now implement `hyper::rt::Executor`.
`hyper::rt::spawn` usage should be replaced with `tokio::task::spawn`.
`hyper::rt::run` usage should be replaced with `#[tokio::main]` or
managing a `tokio::runtime::Runtime` manually.
A Cargo feature `runtime` is added, which is enabled by default, that
includes the following:
- The `client::HttpConnector`, which uses `tokio::net::TcpStream`.
- The `server::AddrStream`, which uses `tokio::net::TcpListener`.
- The `hyper::rt` module, which includes useful utilities to work with
the runtime without needing to import `futures` or `tokio` explicity.
Disabling the feature removes many of these niceties, but allows people
to use hyper in environments that have an alternative runtime, without
needing to download an unused one.
