`pub` `impl Arbitrary`

[skaunov]

I want to use impl Arbitrary downstream in my (utilities) tests.

Should the property tests be arb feature-gated?

I inclinde to no because the tests are not feature-specific and it's a good thing cargo t fails until like --features is added.

#[cfg(any(test, feature = "arb"))]

I considered this approach but having some experience with it shows that syncing versions between the feature and dev.dependencies is a nuisance. Being more naive it doesn't add much (only the working cargo t but it works smoothly when the feature is added).