resume from C-c

[nikomatsakis]

It would be nice if we could easily recover from a C-c and pick up testing where we left off. I am imagining something like this. The initial cargo fozzy command will save bits of data into the work directory, including the list of commits to test, and begin working. You can then later run cargo fozzy resume which will read the list of commits from the disk, probe the work directory to see how far we got, and then keep going.

Probably the default command should error out if there is an "incomplete run" saved to disk, and we can add a cargo fozzy clean command to clear that state out.