chore: refactor simd code in preparation for x86 intrinsics #456

Job	Run time
Build wasm	2m 47s
Run tests on Linux x86 (nightly)	51s
Run tests on Linux arm64	1m 2s
Run tests on Linux x86 (nightly, no default features)	54s
Run tests on Linux x86 (stable, no default features)	52s
Run tests on macOS arm64 (Apple M1)	1m 14s
Run tests on Linux armv7	1m 3s
Run tests on Linux x86 (stable)	47s
	9m 30s

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