buffer: optimize buffer.concat performance #3257

Job	Run time
Build slim tarball	2m 40s
aarch64-linux: with shared libraries	29m 4s
x86_64-linux: with shared libraries	34m 34s
x86_64-darwin: with shared libraries	43m 42s
aarch64-darwin: with shared libraries	35m 55s
	2h 25m 55s

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