Simplify guard form using call-in-continuation #413

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Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	30m 4s
ci (a6osx, macos-13)	15m 46s
ci (ta6osx, macos-13)	24m 42s
ci (arm64osx, macos-14)	8m 45s
ci (tarm64osx, macos-14)	10m 58s
ci (i3le, ubuntu-22.04)	12m 47s
ci (ti3le, ubuntu-22.04)	15m 46s
ci (a6le, ubuntu-22.04)	9m 11s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	12m 1s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 27s
ci (i3nt, windows-2022, vs)	11m 23s
ci (ti3nt, windows-2022, vs)	15m 2s
ci (a6nt, windows-2022, vs)	12m 11s
ci (ta6nt, windows-2022, vs)	14m 47s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 20s
ci (ta6le, ubuntu-22.04, test-more, more)	58m 54s
	4h 34m 4s

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