Restore poll_*_vectored methods

[yoshuawuyts]

#87 removed the vectored implementations because mio doesn't support std's ioslice yet, which is used in the current AsyncRead and AsyncWrite traits. We should restore these implementations once mio supports it.

Refs

• https://docs.rs/mio/0.6.16/mio/net/struct.TcpStream.html#method.read_bufs
• futures-preview 0.16 #87

[taiki-e]

tokio-rs/mio#957