Optimize StructArray views

[mourner]

A small experiment to see if potential optimizations to StructArray (which is on a hot path) could make a difference to load time:

• Do not create uint8 view when members don't use it; instead, use the smallest used view for resizing array buffers.
• Cache view variables in emplace.

Will report back once the benchmarks finish.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• post benchmark scores
• manually test the debug page
• 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></changelog>

[mourner]

@ansis here's the benchmark run for these https://sites.mapbox.com/benchmap-js-staging2-site/runs/436 — not sure if worth landing, but it does decrease total number of objects in memory.