Inline `hash_mul_`. #673

Job	Run time
Build and test cmake:gcc9	44s
Build and test cmake:clang15	34s
Build and test cmake:clang15:asan	1m 7s
Build and test cmake:clang15:tsan	32s
Build and test cmake:clang15:ubsan	55s
Build and test cmake:qemu-arm-neon-gcc	1m 14s
Build and test cmake-osx:clang	39s
Build and test cmake-osx:gcc	39s
Build and test cmake-win64:msvc-rel	1m 16s
Build and test cmake-win64:msvc-dbg	1m 8s
Build and test fuzz:clang15	20s
Build and test python3.10:clang15	1m 39s
Build and test python3.10-win	46s
Build and test maven	26s
Build and test bazel:root	27s
Build and test bazel:go	1m 12s
Build and test bazel:java	2m 10s
Build and test bazel:research	44s
Build and test bazel-osx:root	1m 18s
Build and test bazel-osx:go	1m 52s
Build and test bazel-osx:java	3m 25s
Build and test bazel-osx:research	1m 27s
Build and test bazel-win:root	1m 33s
Build and test bazel-win:java	3m 58s
Build and test bazel-win:research	1m 4s
	31m 9s

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