Refactor SymbolBucket#addFeature

[jfirebaugh]

Shorter and easier to understand.

benchmark	master ab83620	refactor-symbol-bucket-add-feature b818121
map-load	131 ms	98 ms
style-load	79 ms	82 ms
buffer	989 ms	988 ms
fps	60 fps	60 fps
frame-duration	4 ms, 0% > 16ms	4.1 ms, 0% > 16ms
query-point	0.87 ms	0.92 ms
query-box	74.26 ms	74.92 ms
geojson-setdata-small	7 ms	5 ms
geojson-setdata-large	223 ms	268 ms

Launch Checklist

• briefly describe the changes in this PR
• post benchmark scores
• manually test the debug page

[mollymerp]

yesss love this simplification. do you think the 25% speed up in the map-load bench is really a thing or is it within the normal variability of the bench marks? whoa if true 👏 🎈 ⚡️

[mourner]

@mollymerp no, this particular benchmark is not useful at this time because it varies too much regardless of the changes. The second run is always smaller.