The public API of the taskchampion crate now contains the expected parts and no more, and has some better documentation. This moves the crate's external `tests/` into internal tests, as the TaskDB is no longer exposed as part of the crate API.
- taskchampion -- core implementation of a replica - taskchampion-cli -- command-line interface - taskchampion-sync-server -- server implementation (not much yet!)
