Simplify indexWord8Array and writeWord16Array #207

Job	Run time
i386	5m 57s
arm (arm32v7)	4m 33s
build (ubuntu-latest, 8.0.2, 3.8.1.0)	3m 27s
arm (arm64v8)	3m 35s
darwin_arm (self-hosted, macOS, ARM64, 8.10.7)	5m 44s
build (ubuntu-latest, 8.2.2, 3.8.1.0)	2m 50s
darwin_arm (self-hosted, macOS, ARM64, 9.2.6)	55m 48s
build (ubuntu-latest, 8.4.4, 3.8.1.0)	3m 41s
darwin_arm (self-hosted, macOS, ARM64, 9.4.4)	47m 53s
build (ubuntu-latest, 8.6.5, 3.8.1.0)	2m 34s
build (ubuntu-latest, 8.8.4, 3.8.1.0)	2m 34s
build (ubuntu-latest, 8.10.7, 3.8.1.0)	3m 2s
build (ubuntu-latest, 9.0.2, 3.8.1.0)	3m 39s
build (ubuntu-latest, 9.2.7, 3.8.1.0)	6m 17s
build (ubuntu-latest, 9.4.5, 3.8.1.0)	8m 11s
build (ubuntu-latest, 9.6.1, 3.8.1.0)	6m 15s
build (macOS-latest, 9.4.5, 3.8.1.0)	14m 32s
build (macOS-latest, 9.6.1, 3.8.1.0)	14m 33s
build (windows-latest, 9.4.5, 3.8.1.0)	16m 25s
build (windows-latest, 9.6.1, 3.8.1.0)	17m 33s
	3h 49m 3s

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