Custom transports don't have access to serialize

[isaac-mcfadyen]

Hey! 👋

Awesome library, thanks for all of the work on it.

Small issue I'm running into when trying to make a custom transport.

• The chitchat::serialize module is marked as pub(crate), which means you can't serialize a ChitchatMessage outside of the crate.
• Additionally, since ChitchatMessage isn't marked as Serde's Serialize/Deserialize, there's actually no way to serialize messages for a custom transport.

This seems like an easy fix unless there's a specific reason not to: just remove the pub(crate) and leave pub on chitchat::serialize.

[melbourne2991]

Additionally, the transport & socket traits are public but to implement them you would need to re-implement serialization.

I'll create a PR for this 🤞