Add reduction for add1/sub1 for fixnums in cptypes #430

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	29m 37s
ci (a6osx, macos-13)	14m 2s
ci (ta6osx, macos-13)	19m 27s
ci (arm64osx, macos-14)	9m 51s
ci (tarm64osx, macos-14)	11m 53s
ci (i3le, ubuntu-22.04)	12m 43s
ci (ti3le, ubuntu-22.04)	16m 17s
ci (a6le, ubuntu-22.04)	9m 6s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	12m 17s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 24s
ci (i3nt, windows-2022, vs)	12m 27s
ci (ti3nt, windows-2022, vs)	14m 35s
ci (a6nt, windows-2022, vs)	12m 20s
ci (ta6nt, windows-2022, vs)	15m 0s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 0s
ci (ta6le, ubuntu-22.04, test-more, more)	58m 49s
	4h 29m 48s

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