Ticket #285 is closed and most of this is stable now we think.
(There are still a couple of stability warnings for specific types in
tor-config, which aren't exposed at the arti-client level.)
To do this at all neatly, I had to split out `tor-config` from
`arti-config` again, and putting the lower level stuff (paths,
builder errors) into tor-config. I also changed our use of
derive_builder to always use a common error type, to avoid
error type proliferation.
Emil Engler
Ian Jackson
Dimitris Apostolou
Nick Mathewson