io: add async BufReader/BufWriter

[taiki-e]

Closes #1437

Refs: rust-lang/futures-rs#1573, rust-lang/futures-rs#1608

[carllerche]

Thanks 👍 great work as always.

[ohmyarch]

@taiki-e Thanks! I can't wait to write some test code, but it didn't compile:

use tokio::{
    io::{AsyncBufReadExt, BufReader},
    net::TcpStream,
    prelude::*,
};

async fn foo() {
    let mut socket = TcpStream::connect("some address".parse().unwrap()).await.unwrap();
    let (reader, writer) = socket.split();
    let mut reader = BufReader::new(reader);
    reader.read_unitl(b'\n', vec![]).await.unwrap();
}

error[E0599]: no method named read_unitl found for type tokio_io::io::buf_reader::BufReader<tokio_tcp::split::TcpStreamReadHalf> in the current scope

[taiki-e]

@ohmyarch That's typo...

-    reader.read_unitl(b'\n', vec![]).await.unwrap();
+    reader.read_until(b'\n', vec![]).await.unwrap();

[ohmyarch]

@taiki-e Oh my God! Sorry, I am so stupid😂. It seems that I was spoiled by g++ (which reminds me of typo).