Encapsulate usage of config-rs inside tor-config.

Nick Mathewsonrequested to merge
nickm/arti:encapsulate_config_rs into main

This is a chopped-down version of !1989 (closed) that does not port to figment, and only keeps the other refactorings. It's still an improvement, inasmuch as it hides our usage of config-rs. I'm hoping we can merge this while we're stalled deciding about moving to figment.

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