Further configuration refactoring
This branch implements more of the planned refactoring for #84 (closed).
It makes builders across crates more consistent, and moves TorClientConfig and ArtiConfig towards a more similar layout. (In fact, the layout should match completely except for the items that only ArtiConfig has.) It cleans up a few other inconsistencies and infelicities.
It also implements the meta-builder pattern for TorClient config, and ensures that every builder can be reconstructed from its underlying configuration object.
(FWIW, I'm trying to learn how to use a modern IDE, and did this branch using it. Please let me know if the commits are messed up somehow.)