WIP: Fix function rewriter for domains `Nat` and `Int`

[shonfeder]

Closes #2179

WIP diagnosing and working on fix for a rewriter error causing any functions of the form [x \in Nat |-> ...] or [x \in Int |-> ...] to become 0.

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• [ ] Tests added for any new code
• [ ] Ran make fmt-fix (or had formatting run automatically on all files edited)
• [ ] Documentation added for any new functionality
• [ ] Entries added to ./unreleased/ for any new functionality

[shonfeder]

@konnov said we shouldn't be allowing functions with infinite domain at all, and that he has a fix in mind, so will leave this until we can sync on Monday.

[shonfeder]

super stale here, and we've the issue to track it.