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In writeBytes return actual numBytesWritten #164

Job	Run time
windows (aarch64)	1m 9s
ubuntu (x86, g++-multilib)	1m 43s
windows (x86_64)	1m 21s
macos (aarch64, 11, MacOSX11.0.sdk)	1m 6s
ubuntu (armhf, g++-arm-linux-gnueabihf)	37s
ubuntu (ppc64, g++-powerpc64le-linux-gnu)	43s
ubuntu (mingw64, g++-mingw-w64-x86-64)	1m 29s
ubuntu (x86_64, g++-)	1m 42s
ubuntu (mingw32, g++-mingw-w64-i686)	1m 28s
ubuntu (aarch64, g++-aarch64-linux-gnu)	1m 41s
macos (x86_64, 10.9, MacOSX10.9.sdk)	28s
ubuntu (mingwaarch64, clang)	1m 47s
ubuntu (riscv64, g++-riscv64-linux-gnu)	45s
	15m 59s

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