Jsperf compare to `new Set()`

using unordered-set mutates the object by setting a own property `_index ` on the objects
I guess this have some performances penalties?

I'm betting the native Set dose some some similar thing but use pointers instead?
my bet is that `new Set()` is faster

Review the unordered-set implementation that adds the `_index` property, then create or run a jsperf comparison with native `Set`. Done means the benchmark results clarify the performance difference and account for the mutation concern.

Written by the indexing model from the issue text.