elliptic-curve: leverage `CtOption::into_option` #2358

Job	Run time
test (1.65.0)	1m 36s
test (stable)	13s
build (1.65.0, thumbv7em-none-eabi)	1m 34s
build (1.65.0, wasm32-unknown-unknown)	1m 28s
build (stable, thumbv7em-none-eabi)	22s
build (stable, wasm32-unknown-unknown)	10s
	5m 23s

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