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.