buffer: optimize buffer.concat performance #3142

Job	Run time
Build slim tarball	1m 34s
aarch64-darwin: with shared libraries	47m 24s
x86_64-darwin: with shared libraries	1h 2m 13s
aarch64-linux: with shared libraries	37m 11s
x86_64-linux: with shared libraries	47m 30s
	3h 15m 52s

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