Port from config-rs to figment
I am not sure we should actually do this. Having gotten the work done, I don't know whether it's an improvement or not. I think the source for figment is a bit nicer, and its approach to errors seems somewhat better (see #1267 (closed)), but there are still some issues where I wish figment did things differently.
(In particular, see https://github.com/SergioBenitez/Figment/issues/94 .)
Closes #1267 (closed).
Closes #1268 (closed).
If we choose not to merge this, we should IMO at least merge the earlier parts of this branch, which make config
no longer part of the public API for the tor-config
crate.