RFC: Proposed config taxonomy changes

Tickets #458 (closed) and #285 (closed) request a review of our config taxonomy.

Now that arti-example-config.toml is tested to correspond to the config interpretation code, it is convenient to review the taxonomy there. I have done this. It looks pretty good to me, to be honest. I propose the following two changes:

• dns_port and socks_port ought to become dns.port and socks.port, so that in the future we can have socks.listen_address or something.
• download_* ought to become download.* (#489 (closed))

Currently the CI will fail since I have only changed the example to demonstrate the proposal, not any code. We should discuss these proposed changes, and any others, and then when agreed they can be implemented.

CC @nickm @eta: Please review not only my proposed changes, but the proposed new file as a whole.