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.
Mainly, drop the suggestion to fetch submodules, and explain what
more things do.
(The only submodule we have now is the fuzzing corpora, which most
people won't want.)
(I am not 100% convinced that we should keep 100% of the
instructions for working with git in the long term. Do many other
projects have these? Is there a tutorial we should link to
instead?)