BlueZ bindings without the fuss

Or rather, with my kind of fuss.

Goals:

don't require tokio (there's an optional feature for tokio support you can enable)
don't expose unstable dependencies in public API (including #[async_trait], the futures crate, and zbus)
- in fact the public API exposes nothing that isn't available in libstd
don't have unreasonably many dependencies
- this library doesn't currently deliver on this goal, zbus pulls in a lot of dependencies