Add reduction for add1/sub1 for fixnums in cptypes #427

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	30m 10s
ci (a6osx, macos-13)	15m 45s
ci (ta6osx, macos-13)	19m 16s
ci (arm64osx, macos-14)	9m 36s
ci (tarm64osx, macos-14)	10m 26s
ci (i3le, ubuntu-22.04)	12m 48s
ci (ti3le, ubuntu-22.04)	16m 4s
ci (a6le, ubuntu-22.04)	9m 2s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	12m 24s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 13s
ci (i3nt, windows-2022, vs)	12m 26s
ci (ti3nt, windows-2022, vs)	14m 50s
ci (a6nt, windows-2022, vs)	12m 41s
ci (ta6nt, windows-2022, vs)	15m 0s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 6s
ci (ta6le, ubuntu-22.04, test-more, more)	58m 7s
	4h 29m 54s

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