Implement continuous verification of guppy properties

[sunshowers]

We have code to test various properties that we expect to be true for every dependency graph modeled by guppy. At the moment this code is buried in our test suite and only used for a set of known fixtures. It would be cool to write a tool that can continuously verify a repository or crate on crates.io. Here's how it might be done:

• Extract out the test code into a separate crate.
• Use that crate as the basis for a new CLI tool that verifies a workspace (similar to and extending Implement framework for comparing guppy's results to cargo #83).
• Figure out how to make this tool run continously across many repos and over time.