Implement Resumability

[artrixdotdev]

This is an issue tracker for tracking our progress on implementing resumable torrents and saving the state of the engine to be used later.

The goal:

• Create a EngineExport struct that contains all the relevant state of the engine needed for resuming later
• Have EngineExport implement Serialize and Deserialize from serde so it can be coerced into any data format (json, toml, etc.)
• Implement the actual resumability logic when the user passes a EngineExport struct into the engine
• Create a usable engine.export and Engine::from_export(export) API or something of that nature.