When and what to report as resolution progresses

[uranusjr]

A stub issue to remind myself. Will first need to collect what exactly Cargo does first. A sensible default implementation using logging might be a good idea here as well.

[pradyunsg]

suggestion: it might also be worth looking into https://github.com/CocoaPods/Molinillo and how CocoaPods uses it's "generic dependency algorithm".

[uranusjr]

When debugging Molinilo fixtures I constantly add logging and breakpoints at three locations:

• New candidate introduced (pinned).
• Retracting a previously-pinned candidate due to backtracking.
• A requirement exhausted all possible candidates, but cannot find a pin (which will trigger backtracking later).

So they are likely good reporting points.