feat: add op for creating new/empty list

[acl-cqc]

Constants cannot refer to type variables, so you can't have a constant for an empty List<T>...

• Add ListOp::new to handle this
• Constant-fold it to an empty ListValue if the element type has no type_vars
• (Most of the PR) add new functions (kept pub(crate) for now) for whether a Type, TypeArg or ExtensionSet contains variables.

I'm a bit, ugh, do we have to do this; when we move to hugr-model, we'll have a better way, so we'll end up redoing this again. Hence I've not written any tests yet, but will do if we think we want this....?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.38%. Comparing base (fc609a2) to head (82e27ea).
Report is 6 commits behind head on main.

Additional details and impacted files

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  Coverage   86.38%   86.38%           
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  Files         166      166           
  Lines       30063    30063           
  Branches    26975    26975           
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  Hits        25969    25969           
  Misses       2558     2558           
  Partials     1536     1536           

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[mark-koch]

I think it would be worth doing this, but I don't feel strongly. Also, I'm not super familiar with the discussion around hugr model, so a second opinion might be good.

In case we add the op, we should also implement the llvm lowering