Add pre-publish semver check with the version being published as well

[obi1kenobi]

Currently, before cargo publish, the new version of cargo-semver-checks is tested by running the most-recent-prior version of cargo-semver-checks on its API changes. This makes sure we don't break semver for any public APIs, in preparation for #67.

It would be nice to test the new release with itself as well, for two reasons:

• the new release may contain new checks which could detect and prevent new kinds of semver problems
• it's a low-cost way of getting more "real-world" test coverage of the new release before it's published on crates.io