ctutils: use new `cmov` trait impls; remove `unsafe` #2183

Job	Run time
test (1.85.0)	15s
build (1.85.0, wasm32-unknown-unknown)	18s
test (stable)	15s
build (stable, thumbv7em-none-eabi)	24s
build (stable, wasm32-unknown-unknown)	27s
minimal-versions / minimal-versions	27s
build (1.85.0, thumbv7em-none-eabi)	15s
	2m 21s

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