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Even though it doesn't do much yet, the arti CLI will end up being able to do a multitude of different things, if it's going to ever properly replace the C tor implementation (e.g. hidden service key management, checking its config file, running as a relay, ...).

It's probably a good idea to move over to a command line argument parser that uses subcommands, instead of our current approach, to leave room in for extending it in this way. clap seems to be somewhat of an ecosystem standard, so we should just port the current code to use that.