Integrate policy expressions code

[alilleybrinker]

This is still a work-in-progress, and nothing is actually used yet, but this at least moves over the draft implementation of policy expressions from the external experimental repo I'd made to enable integration with Hipcheck.

The other thing I did was organize the Cargo.toml file for the hipcheck crate, particularly to put all dependencies in alphabetical order, as they had been at one point previously.

Resolves #260

[alilleybrinker]

Looks like I'll need to rebase to get this working.