A build script reaching into your .git/hooks/ and modifying them
nonconsensually was a bit of a horrifying concept, and also made it hard
to build arti with the feature disabled. Remove this crate, and replace
it with manual instructions on how to install the hooks in
CONTRIBUTING.md.

Explain how to report bugs and how get accounts on the bugtracker;
add link to the troubleshooting document.

Part of #277

Eventually we may need a separate HACKING.md

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.

Replace the trace config file option with a more flexible trace_filter
and expose the same via the ARTI_LOG environment variable.
If ARTI_LOG is set it will override the setting in the config file.

Example:

ARTI_LOG="info,tor_proto::channel=trace" ./target/debug/arti

The syntax is described here
https://docs.rs/tracing-subscriber/0.2.20/tracing_subscriber/filter/struct.EnvFilter.html#directives

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?)

Closes #151.

Closes #106