Improve performance of symbol layers with identical or no text

[mourner]

Closes #7973. There was an accidental quadratic behavior in CrossTileSymbolIndex (responsible for deduplicating symbols across tiles) when a map had a ton of symbols with the same text (or no text, just an icon).

This PR changes the logic so that instead of indexing symbol instances by key (which is a numeric hash of its text), we make a KDBush index and search for duplicate symbols spatially. KDBush is already included as a dependency of Supercluster, so it doesn't increase the bundle size, and it's pretty fast, so the approach should work well for both the "lots of identical icons" (added in the debug/symbols-generated.html debug page) and "lots of different text labels" (e.g. Streets) cases.

In theory, this should also improve memory footprint for Streets because we avoid creating thousands of small arrays and a big object with key to array mapping — KDBush is much more memory-efficient. We should run a benchmark before merging to make sure it doesn't accidentally regress anything.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• post benchmark scores
• manually test the debug page
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port — @alexshalamov this affects Native too because it has the same logic.
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Improve performance of symbol layers with identical or no text</changelog>

[alexshalamov]

Nice! cc/ @endanke

[endanke]

@alexshalamov I'm already on it :) (picked up the parity ticket)

[mourner]

Checked the benchmark runs and they're looking OK, so I'm merging. There's an uptick in array buffer count, but that should be OK because at the same time the total amount of objects is reduced. I'll also consider a further improvement in memory footprint by landing mourner/kdbush#32 and then reorganizing the logic here to hold key/crossTileID data in flat numeric arrays instead of an array of objects.