ctutils: use new `cmov` trait impls; remove `unsafe` #2176

Job	Run time
test (stable)	16s
build (1.85.0, wasm32-unknown-unknown)	17s
test (1.85.0)	17s
build (stable, thumbv7em-none-eabi)	18s
minimal-versions / minimal-versions	27s
build (1.85.0, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	14s
	2m 3s

Provide feedback