Remove the Target type from Kore.Equation

[andreiburdusa]

As #2120 (comment) describes, but with an extra case for when swapping a pair produces a collision of keys.

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Review checklist

The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.

• Summary. Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
• Documentation. Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
• Tests. Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
• Clean up. The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!

[ttuegel]

I added more documentation, but I also (as reviewer) need to tend to a couple of clean-up item.s

[ttuegel]

I did some clean up and spent a few hours trying to catch what I thought must be a latent bug, before I finally convinced myself that the bug I imagined was impossible. I added a comment to explain my reasoning. @andreiburdusa Please review what I refactored. You can merge it when you are satisfied.