reduce memory use of pbchunk conversion #244

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	28m 8s
ci (a6osx, macos-12)	16m 48s
ci (ta6osx, macos-12)	19m 58s
ci (arm64osx, macos-14)	8m 10s
ci (tarm64osx, macos-14)	10m 11s
ci (i3le, ubuntu-22.04)	11m 30s
ci (ti3le, ubuntu-22.04)	14m 37s
ci (a6le, ubuntu-22.04)	8m 51s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	12m 4s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 14s
ci (i3nt, windows-2022, vs)	11m 10s
ci (ti3nt, windows-2022, vs)	14m 43s
ci (a6nt, windows-2022, vs)	11m 36s
ci (ta6nt, windows-2022, vs)	14m 50s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 39s
	3h 24m 29s

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