Bump arti to 0.8.3, and rework crate dependencies

• We update the arti_client crate to 0.8.3, in order to get access to the API added in arti!1073 (merged).
• We also remove dependencies on the crate from anything other than onion_tunnel, and ensure all needed types are exported from that crate instead.
  • SQLite now gets compiled as static using the arti_client feature flag, instead of adding an unused direct dependency.
  • In particular, why were we even depending on async-std?