Make hax-lib optional even when using trait and impl

[karthikbhargavan]

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.

[W95Psp]

This will be fixed by:

• feat(hax-lib): intro. hax feature #983
  This PR adds a hax feature that makes the hax-lib dummy, thus enabling shipping hax-lib as a dependency in projects.