The `Accept` trait is used by the server types to asynchronously accept
incoming connections. This replaces the previous usage of `Stream`.
BREAKING CHANGE: Passing a `Stream` to `Server::builder` or
`Http::serve_incoming` must be changed to pass an `Accept` instead. The
`stream` optional feature can be enabled, and the a stream can be
converted using `hyper::server:🉑:from_stream`.