Support for a binary Websocket protocol

[jmcphers]

Kallichore's Websocket protocol is modeled after that of the Jupyter kernel gateway; that is, it sends text messages over the websocket that are well-formed JSON.

{
    "channel": "iopub",
    "header": {
        "msg_type": "example"
    },
    "content": {
        "example": true
    }
}

This text-based representation has a few upsides:

• it makes debugging a lot easier since you can examine the message and its contents as simple strings
• it makes it easier to compose valid socket messages using high-level tools and in test frameworks

However, it also has a few downsides:

• it is not very space efficient
• it is unfriendly to raw binary data such as the message buffers (which we are currently base64 encoding)
• it is slightly more computationally expensive since everything needs to be serialized on the backend and then parsed on the front end

For these reasons, the official Jupyter Server uses a binary-based WebSocket protocol. We could support exactly the same one, as an option, if needed:

https://jupyter-server.readthedocs.io/en/latest/developers/websocket-protocols.html