fix(access): properly seal access traits

[mkroening]

Before, anyone could just implement Access, Readable, Copyable, and Copyable because _private() was already available as default implementation, removing the need to construct _Private (impossible because of #[non_exhaustive]).

This PR properly seals the affected traits as described in the Rust API Guidelines: Sealed traits protect against downstream implementations (C-SEALED)—Future proofing—Rust API Guidelines.

While this is a soundness issue, I don't think this is yank-worthy.

Note that _private was not hidden in the docs, so removing that would be a breaking change, strictly speaking, but I doubt that this is a problem in practice.

[phil-opp]

Good catch, thanks! I don't know why I didn't think of the consequences of a default impl...

I think the intention of keeping the traits private was still clear, so I would treat this as a bug fix instead of considering it a breaking change.