Add flatfile DirMgr

This is a first shot at implementing the changes needed for #267 (closed).

• Definition of a DirProvider trait, abstracting the DirMgr. This is not complete, and in particular there is an issue with the client calling bootstrap(): the call cannot be abstracted in the trait, because async fn's are not (yet) supported.

• Changes in TorClient to use a DirProvider instead of a DirMgr. Not complete, due to the above (currently has a nasty hack of keeping a reference to the actual DirMgr).

• Implementation of a minimalist DirProvider called FlatFileDirMgr, that only loads the directory from cache files. Hidden behind a lightarti feature.

• Changes in TorClient to use FlatFileDirMgr under a lightarti feature.

• Addition of a couple of methods to UnvalidatedMdConsensus, to help manipulating the relays in FlatFileDirMgr.

• Addition of FromStr for Authority, to allow us to load an authority from a file. The use of it is actually outside of Arti, in our client code, feeding the authorities to TorClient through configuration.

All comments and suggestions for improvements welcome. 😄 (Given that I am very new to Rust, I am sure I wrote a lot of "unrusty" things 🙃 ).