Use a `Set` for referencers

[clenfest]

When working with elements with many referencers, addReferencer() became very hot.

If you have eg. 100 referencers, each one is added one by one, and each new addition iterates the entire existing referencers list. This could cause quadratic performance.

This changes the _referencers values to be JS Sets. This maintains the same order for algorithms that rely on order.

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[goto-bus-stop]

It actually might be good to convert unionsWhereMember and allImplementations, too. I didn't do it because it's more work. But allImplementations in particular is used for the runtimeTypesIntersect check, which is currently O(n^2), and could be O(n) if allImplementations used a Set. But maybe do that when we have an example query where it is very hot?