Integrate Cargo SemVer Checks

[Carter12s]

https://github.com/obi1kenobi/cargo-semver-checks

Currently we're playing pretty fast and loose with versions. Would be good to actually start versioning each sub-package independently. Being correct about the public APIs of each sub-crate, and following semver a bit more.

Good practice as we eventually try to steer to a 1.0 release (still far away)