elliptic-curve: leverage `CtOption::into_option` #2115

Job	Run time
minimal-versions	0s
no_std (1.75.0, thumbv7em-none-eabi)	21s
no_std (1.75.0, wasm32-unknown-unknown)	15s
no_std (stable, thumbv7em-none-eabi)	18s
no_std (stable, wasm32-unknown-unknown)	8s
test (1.75.0)	20s
test (stable)	11s
	1m 33s

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