Adding feature to verify coherence between local index and mirror

[ricked-twice]

Hi there! :wave:

So here the things, a few days ago, I was using panamax and decided to synchronize my mirror (after maybe one month or two without doing it). What happened is exactly what is described on issue#55 (which is now closed by the author).

To sum up, I had my mirror in an inconsistent state: index was referencing a crate, but this one was not present on my mirror. This happened right after panamax sync /path/to/my/mirror, although panamax stated the sync was a success. As I really needed an up-to-date mirror, I deleted the mirror and ran a fresh new sync. And then it worked, my mirror was coherent with the index.

But, the problem remains. I tried to trigger it again, but without any success. So I decided to do add a quick feature to detect this kind of problem.

Of course do not hesitate to say what you think about my contribution (this is my first one ever :metal: ) and feel free to modify it.

Main changes:

• [x] Added an impl on CrateEntry to be able to obtain reference to crate's name and version.
• [x] Passed visibility of CrateEntry.yanked to pub(crate)
• [x] Adding a module (verify.rs) containing the logic.

[k3d3]

Looks great, thank you!