Right now the sync API needs to use async-std and cannot be used with tokio. This was originally due to some limitations in the tokio API that would require us to acquire a lock each time an operation was executed, but this no longer seems to be the case. This will allow users to avoid having to specify default-features=false when enabling the sync API, which would be a minor ergonomic win. Also, it's possible that the performance of the sync API could improve if using tokio.