Implement old and rel

[tillarnold]

This implements old , rel0 and rel1

• The syntax for calling rel from Rust is rel0(...) instead of rel(...,0) , since the integer needs to be a constant in viper

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Notes for me:

• branch with everything merged in:
  • tillarnold/prusti-dev@till_old_impl2...tillarnold:prusti-dev:till_old_impl2_remove_defid
  • tillarnold/prusti-dev@Aurel300:rewrite-2023...tillarnold:prusti-dev:till_old_impl2_remove_defid

[JonasAlaif]

@tillarnold just fyi, I sent you a message on Zulip about our meeting today

[Aurel300]

Looks good and is mostly doing things in the front-end, i.e. won't clash with other PRs, but I would like to change this to not make the marker kinds so hardcoded, i.e. have an enum Mode { Old, Rel(u32), } somewhere.