Use tokio instead of async-std.

[jsdw]

Jsonrpsee (a dependency) uses tokio under the hood. Would it make sense for subxt to align on this and also use tokio instead of async-std (which is currently only a dev dependency anyway) to avoid pulling in both?

From a quick glacne it looks like it would be a fairly trivial swap; #[async_std::test] tp #[tokio::test] and a few other similarly easy changes.