FTP codec

The only remaining thing to code before we can try our FTP server is the codec. So, let's create a new module for the codec:

mod codec;

In the src/codec.rs file, we'll create our FTP codec:

pub struct FtpCodec;

To create a codec, we must implement the traits Decoder and Encoder. These traits come from the tokio-io crate:

use tokio_io::codec::{Decoder, Encoder};