Add deprecation notice to af_alg_iv::as_slice and trait implementations that depend on it

[eduardosm]

These trait implementations exposed an unsound API (see #1501).

[rust-highfive]

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[eduardosm]

@JohnTitor ping

[JohnTitor]

Given from #1501 (comment) and #1501 (comment), I'd be in favor of removing that implementation but I'd like to take some period for other users to notice. We could add deprecated attr at first and remove stuff after some releases.

[eduardosm]

Looking at what I did with the Debug implementation (print ivlen and ignore iv), would it be acceptable to do the same with the other traits (e.g., PartialEq would only compare ivlen)? This would allow to keep the PartialEq and Hash implementations, but their behavior would change.

[JohnTitor]

So, as discussed in #1892, let's just remove the impls for it. But as I said before, I'd add a deprecation note to make the breaking change less-painful. r=me once it's added instead of removing the impls here.

[eduardosm]

Done, I have re-added the implementations and marked them as deprecated.

[JohnTitor]

@bors r+ Thanks!

[bors]

📌 Commit d73e8a3 has been approved by JohnTitor

[bors]

⌛ Testing commit d73e8a3 with merge 582c5cf...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-10, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12
Approved by: JohnTitor
Pushing 582c5cf to master...