Use `jsonschema` only as a dev-dependency

[Shnatsel]

Depends on #749

Fixes #741

Also add exclude directives to Cargo.toml to cut the tarball size in half.

You'd think you could just stick #[cfg(test)] on a module and call it a day, but #[cfg(test)] isn't actually set for the main crate when running integration tests. Making a separate crate for the test utils is the least clunky option: https://stackoverflow.com/a/44545091/585725