Remove byte-unit dependency in commons public interface

We cannot upgrade this dependency until errors are fixed https://github.com/heroku/buildpacks-ruby/pull/252. We should remove it from the interface entirely.

I'm not 100% sure how to gently do this. In ruby I would accept two different input types for awhile and at runtime issue a deprecation warning. I guess in Rust I just remove/change the type and let the compiler fail, maybe rev a breaking change version on commons.