Consider adding async wrapper

[ecton]

While we are trying to keep BonsaiDb lightweight, it might be interesting to explore creating a lightweight async wrapper. I'm not sure we'd use it within BonsaiDb, but if this wrapper also dealt with the lifetime issues transparently, it could be worth it to switch.

Lifetime issues: spawn_blocking requires 'static lifetimes for the closure being executed. Using something like cryo we can take borrowed arguments and temporary extend the lifetime to static.

Refs #30