elliptic-curve: leverage `CtOption::into_option` #2093

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 33s
build (1.65.0, wasm32-unknown-unknown)	1m 30s
build (stable, thumbv7em-none-eabi)	11s
minimal-versions	37s
test (1.65.0)	1m 35s
build (stable, wasm32-unknown-unknown)	13s
test (stable)	14s
	5m 53s

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