Bump version to 0.8.4 #161

Job	Run time
linux (x86_64-unknown-linux-gnu)	3m 28s
linux-cross (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 28s
macos-aarch64	1s
linux (i686-unknown-linux-gnu)	3m 27s
macos-x86_64	1s
linux-cross (armv7-unknown-linux-gnueabihf, armv7)	3m 27s
musllinux (x86_64-unknown-linux-musl)	3m 27s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 28s
linux-cross (s390x-unknown-linux-gnu, s390x)	3m 28s
musllinux (i686-unknown-linux-musl)	3m 26s
sdist	3m 38s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	3m 28s
linux-cross (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 27s
linux-cross (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 27s
windows (x86_64-pc-windows-msvc, x64)	3m 23s
linux-cross (arm-unknown-linux-musleabihf, arm)	3m 29s
windows (i686-pc-windows-msvc, x86)	3m 26s
windows (aarch64-pc-windows-msvc, x64)	3m 23s
	55m 22s

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