Implement non-async version of proptest-stateful

[nickelization]

The current version of this library is tied to async Rust (and specifically to tokio). You can write tests that don't use any async code, and they'll work fine, but the interface is more complex than it needs to be for use cases like that. It would be great if there were a simpler non-async version of the trait that the framework uses.

Additionally, if there were a non-async trait, we could add a feature flag and make compilation of the async code conditional on that flag. That way, if users don't want to use the async features, they're not forced to pull in all the related dependencies and whatnot.