arti-config should not recapitulate arti-client config sections

Currently, arti-config's ArtiConfig contains entries for each of the arti-client config sections, and ad-hoc formulaic code to convert things back and forth.

This should be replaced by some arrangement that allows the config for the atti CLI program to be combined with the arti-client Tor config, without layers above arti-client knowing about specific bits of the Tor config.

For example, a struct with #[serde(flatten)].

This is part of #285 (closed). See also #314 (closed).