fix(rum-explorer): speed up histogram facets

[akalfas]

Changed the facet lookup algorithm from O(NxM) to O(1) by trading space. For some data it can be from

to

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Commits

* [d087d19](https://github.com/adobe/helix-website/commit/d087d19f34ea5b4c04f989cb7903324bf26bbd5b) :white_check_mark: (latest)