Introduce an experimental benchmarking utility for Arti

The new arti-bench crate does a simple end-to-end benchmark test embedding Arti: it generates some random data (of configurable amount, depending on command-line parameters), and then sends said data back and forth via Arti (which should be configured to use a local Chutney network).

Additionally, the benchmark can also be run via a local SOCKS5 server (in order to benchmark the performance via a local Chutney node, for comparison).

The tests/chutney/arti-bench.sh sets up and tears down Chutney as required to make this work.

This is very much a first cut; there are many things that should eventually get added, such as support for multiple connections, JSON output capabilities, running multiple tests, ...