In this part, we'll implement an incomplete MessagePack-RPC library using the whole tokio stack:

• tokio-io for the codec (see chapter 2.1)
• tokio-proto for the protocol (see chapter 2.2)
• tokio-service for the service (see chapter 2.3)

Why an "incomplete" library? We'll see that using only tokio-proto, we cannot implement support for notifications. But worry not! This is something we'll fix in part 3.