ctutils: add `CtNeg` trait #84

Job	Run time
test-careful	56s
test-miri (x86_64-unknown-linux-gnu)	39s
test-miri (s390x-unknown-linux-gnu)	59s
build (stable, thumbv7em-none-eabi)	11s
build (1.85.0, wasm32-unknown-unknown)	17s
build (stable, wasm32-unknown-unknown)	15s
build (1.85.0, thumbv7em-none-eabi)	15s
test (stable)	11s
minimal-versions / minimal-versions	26s
test (1.85.0)	15s
	4m 24s

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