Improve and future-proof the `arti` CLI

This switches out arti's argument-parsing library with clap, which is a lot more featureful (and very widely used within the Rust ecosystem). We also now use a lot of clap's features to improve the CLI experience:

• The CLI now expects a subcommand (currently, either "help", or "proxy" for the existing SOCKS proxy behaviour). This should let us add additional non-SOCKS-proxy features to arti in future.
• clap supports default values determined at runtime, so the way the default config file is loaded was changed: now, we determine the OS-specific path for said file before invoking clap, so the help command can show it properly.
  • The behaviour of tor_config was also changed; now, one simply specifies a list of configuration files to load, together with whether they're required.
  • That function also way overused generics; this has been fixed.
• Instead of using the ARTI_LOG environment variable to configure logging, one now uses the -l, --log-level CLI option. (The intent is for this option to be more discoverable by users.)
• The proxy subcommand allows the user to override the SOCKS port used on the CLI without editing the config file.