Update the way row keys and column keys are constructed in the pivot function. They are now built in a StringBuilder. Previously where they were constructed the string would be rebuilt with every column added to the key. If the key only consisted of a single column then a reference to the column contents would be retained, This reference would no be counted in the reference count, but would be sufficient to keep an entire record batch from being garbage collected.

Checklist

Dear Author 👋, the following checks should be completed (or explicitly dismissed) before merging.

• ✏️ Write a PR description, regardless of triviality, to include the value of this PR
• 🔗 Reference related issues
• 🏃 Test cases are included to exercise the new code
• 🧪 If new packages are being introduced to stdlib, link to Working Group discussion notes and ensure it lands under experimental/
• 📖 If language features are changing, ensure docs/Spec.md has been updated

Dear Reviewer(s) 👋, you are responsible (among others) for ensuring the completeness and quality of the above before approval.