Improve the layout of crate exports; add runtime convenience functions

This commit addresses multiple problems highlighted by #182 (closed):

• arti-client had some types in its public API that weren't accessible without importing another crate (CfgPath, DataReader, DataWriter). This has been fixed.
  • In addition, the doc comments for DataReader and DataWriter were cleaned up to be of better quality, now that they're public.
• It was impossible to use arti-client without also importing tor-rtcompat. This is now fixed by the addition of two convenience methods: TorClient::bootstrap_with_tokio and TorClient::bootstrap_with_async_std.
• Potentially controversially: tor-rtcompat now returns concrete types from methods like current_runtime, instead of impl Runtime.
  • This was needed in order to actually be able to name the TorClient type that results from using these methods.
  • This does mean we lose API flexibility, but on balance I think this is a good thing, because the API we do have is actually usable...