Add bio support to decouple the ssh client from std::net::TcpStream

[HsuJv]

The main commit is the first one
It adds a generic type to the ssh client, decoupling the structure from TcpStream to IO: Read + Write.
This allows the user to connect to the ssh server using different methods (e.g. WebSocket or some other proxies).
POC at here which works fine.

In order not to break the compatibility of the existing APIs, a new API connect_bio has been added which asks the user to provide its buffered input/output objects. Whereas the type-specific bindings for TcpStream were always established in the old connect API.

impl Session<TcpStream> {
    pub fn connect<A>(&mut self, addr: A) -> SshResult<()>
    where
        A: ToSocketAddrs,
    {
        if self.user_info.is_none() {
            return Err(SshError::from("user info is none."));
        }
        // 建立通道
        let tcp = TcpStream::connect(addr)?;
        // default nonblocking
        tcp.set_nonblocking(true).unwrap();

        log::info!("session opened.");
        self.connect_bio(tcp)
    }
}


impl<IO> Session<IO>
where
    IO: Read + Write,
{
    pub fn connect_bio(&mut self, stream: IO) -> SshResult<()> {
        if self.user_info.is_none() {
            return Err(SshError::from("user info is none."));
        }
        // 建立通道
        self.client = Some(Client::<IO>::connect(
            stream,
            self.timeout_sec,
            self.user_info.clone().unwrap(),
        )?);
        log::info!("session opened.");
        self.post_connect()
    }
}

All the old examples work fine.
Add a new example to show how bio works.

[1148118271]

I'll test out the old examples locally and have a new 0.2.2 release soon.