Fix hax compilation of crates relying on classification traits

[maximebuyse]

The fix is to use hax_lib::exclude instead of cfg(not(hax)) to exclude the unsupported items from hax extraction, without breaking compilation under the hax cfg.

This PR replaces #1263

[skip changelog]

[franziskuskiefer]

lgtm. @jschneider-bensch do we have other crates where we should fix this? Also, should we add a dummy crate (or get #1259 in) to have a test in the repo to make sure we don't break this again?

[jschneider-bensch]

do we have other crates where we should fix this? Also, should we add a dummy crate (or get #1259 in) to have a test in the repo to make sure we don't break this again?

It is worth checking to avoid dependency trouble in the future, for sure. We should get #1259 as well.