buffer: optimize buffer.concat performance #3132

Job	Run time
Build slim tarball	1m 33s
aarch64-linux: with shared libraries	35m 10s
x86_64-darwin: with shared libraries	59m 53s
aarch64-darwin: with shared libraries	37m 21s
x86_64-linux: with shared libraries	39m 25s
	2h 53m 22s

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