chore: unify v128 representation #512

Job	Run time
Build wasm	2m 5s
Run tests on Linux x86 (nightly)	1m 18s
Run tests on Linux x86 (stable, no default features)	58s
Run tests on Linux arm64	4m 9s
Run tests on macOS arm64 (Apple M1)	1m 4s
Run tests on Linux x86 (nightly, no default features)	1m 19s
Run tests on Linux x86 (stable)	1m 0s
Run tests on Linux armv7	4m 1s
	15m 54s

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