Support the new binary serialization built into Phoenix

[myobie]

See https://github.com/phoenixframework/phoenix/blob/v1.5.8/lib/phoenix/socket/serializers/v2_json_serializer.ex

We will also, over time, switch over to their style of messages where the sizes come first.

• Support encoding and encoding both ours and theirs on the server – https://github.com/shareup/poc-server/issues/53
• Support encoding and encoding both ours and theirs in the swift client (this)
• Support encoding and encoding both ours and theirs in the js client – https://github.com/shareup/apps-for-browser/issues/63