Make Crater usable for library authors as an API stability test case

[yaahc]

Crater is essentially a test case for rustlang to guarantee that it doesn't introduce any unexpected breaking changes. It would be helpful if crater could be generalized into a test case for ANY library, not just for core / std / etc. That way a library author could kick off a small crater run that does an A-B test with their old and new release with all of their reverse dependencies and they can hopefully catch accidentally introduced breaking changes.

[hdevalence]

Hi, I'd be interested in helping with this! I don't know about all of the parts of crater that need to be changed, but if someone pointed me at issues I would be happy to do the work.

[pietroalbini]

@hdevalence the next one needed is https://github.com/rust-lang/crater/issues/443