Simplify guard form using call-in-continuation #414

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	30m 6s
ci (a6osx, macos-13)	15m 35s
ci (ta6osx, macos-13)	22m 28s
ci (arm64osx, macos-14)	9m 4s
ci (tarm64osx, macos-14)	11m 41s
ci (i3le, ubuntu-22.04)	12m 44s
ci (ti3le, ubuntu-22.04)	16m 4s
ci (a6le, ubuntu-22.04)	9m 7s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	13m 7s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 17s
ci (i3nt, windows-2022, vs)	11m 23s
ci (ti3nt, windows-2022, vs)	14m 31s
ci (a6nt, windows-2022, vs)	12m 12s
ci (ta6nt, windows-2022, vs)	15m 7s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	12m 58s
ci (ta6le, ubuntu-22.04, test-more, more)	59m 16s
	4h 33m 40s

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