For tasks and async functions perform serde off the main thread

For simplicity, async functions and tasks perform serde on the main thread. However, it would perform better to perform this blocking operation on another thread.

Maybe `JsonString`? It would act exactly like `Json`, but stop at the intermediary string. It will need some special handling for `Err` on `serde_json::to_string`. Maybe a `TryFrom` implementation and a `neon::types::extract::Error`.