Make hax-lib optional even when using `trait` and `impl`

[karthikbhargavan]

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The recommended style for projects using hax is to make the dependence on hax-lib optional by guarding all uses of hax attributes with cfg_attr(hax, ...). However, this does not work with the hax_lib::attributes macro that we put on trait and impl. We should try to find a solution that works in this case as well.