Middlend Fixes

[DCupello1]

This pull request is for some middlend fixes/changes that were done in the Rocq backend (#207) and are now being backported.

Fixes include:

• Exposing wf relations, uncase removal projection functions, type families via hints in order to produce a better output in rocq.
• In undep, made List IterE's with no iteration into a ListE in order for validation to work.
• ImproveIds now actually tracks atoms in order to disambiguate them from variables.
• New functionality in free (inter func)
• New functionality in iter (now has visit arg func)
• New functionality in walk (now has collect path func)
• Small fix in sub which was missing a traversal in the expression of the coercion function.
• After IL change, improveids did not have the correct transformation for mixops. This has now been fixed, along with changes to transforming ids with '#' which was added with the new fresh variables generation library.
• Improveids: Added small fix for LetPrs to work.
• Typefamilyremoval: Small fix to generated types to not clash with the base tf type.

[DCupello1]

Rebased this to account for new IL changes. Still leaves improve ids not entirely fixed, will be done in a separate pull request.

[DCupello1]

@nomeata could you look into this PR please? Since the IL refactor is done, these PRs can finally go in.

This one in particular is just really small changes so it shouldn't take long I hope.

[nomeata]

Hmm, I've paged out spectec stuff a bit these days, so I'm not eager and probably not able to give a good review. Maybe someone else whose currently playing with this could?

[DCupello1]

Ah no worries then!