Spawned runtime

Runtime wrapper to remove dependencies from code. Using this library will allow to set a tokio runtime (or any other runtime, once implemented) just by changing the enabled runtime feature. We may implement a deterministic version based on comonware.xyz's runtime: https://github.com/commonwarexyz/monorepo/blob/main/runtime/src/deterministic.rs

Currently, only a very limited set of tokio functionality is reexported. We may want to extend this functionality as needed.