ctutils: rename `Choice::from__nonzero` => `from__nz` #1995

Job	Run time
test (1.85.0)	15s
test (stable)	14s
minimal-versions / minimal-versions	25s
build (stable, wasm32-unknown-unknown)	11s
build (1.85.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	9s
build (1.85.0, thumbv7em-none-eabi)	16s
	1m 49s

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