Since these shell-variables are hardwired to use org.torproject.Arti as
the program name, it isn't appropriate to call them "app-specific".
If we someday reinstate APP_FOO, it should be based on a user-provided
application name.
Now every section that the two configuration objects share has the
same type and name. This should help us in documenting our configuration
in a way that doesn't confuse people.
There is still lots of API work to go.
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.
Nick Mathewson