ctutils: constant-time selection and equality testing #8

Job	Run time
test-careful	55s
test (stable)	16s
test (1.85.0)	36s
build (stable, wasm32-unknown-unknown)	14s
minimal-versions / minimal-versions	19s
build (1.85.0, thumbv7em-none-eabi)	14s
build (stable, thumbv7em-none-eabi)	10s
build (1.85.0, wasm32-unknown-unknown)	18s
	3m 2s

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