Auto merge of #120024 - Mark-Simulacrum:fast-union-merge, r=cjgillot

Merge into larger interval set

This reduces the work done while merging rows. In at least one case (#50450), we have thousands of union([range], [20,000 ranges]), which previously inserted each of the 20,000 ranges one by one. Now we only insert one range into the right hand set after copying the set over.

This cuts the runtime of the test case in #50450 from ~26 seconds to ~6 seconds locally, though it doesn't change the memory usage peak (~9.5GB).

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