optimize getter/setter creation for StructArray elements

[pirxpilot]

minor optimization in accessor creation code

[jfirebaugh]

benchmark	master b93bc45	pirxpilot-optimize-struct-array 991275f
map-load	110 ms	95 ms
style-load	68 ms	61 ms
buffer	940 ms	916 ms
fps	59 fps	59 fps
frame-duration	5.6 ms, 0% > 16ms	5.6 ms, 0% > 16ms
query-point	0.73 ms	0.74 ms
query-box	40.98 ms	40.04 ms
geojson-setdata-small	2 ms	3 ms
geojson-setdata-large	120 ms	95 ms

[jfirebaugh]

Thank you @pirxpilot!