nixos: overhaul Tor module

This overhauls the Tor module in a few ways:

  - Uses systemd service files, including hardening/config checks
  - Removed old privoxy support; users should use the Tor Browser
    instead.
  - Remove 'fast' circuit/SOCKS port; most users don't care (and it adds
    added complexity and confusion)
  - Added support for bandwidth accounting
  - Removed old relay listenAddress option; taken over by portSpec
  - Formatting, description, code cleanups.

Signed-off-by: Austin Seipp <aseipp@pobox.com>