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.
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