elliptic-curve: leverage `CtOption::into_option` #2096

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 31s
build (1.65.0, wasm32-unknown-unknown)	1m 31s
minimal-versions	37s
build (stable, thumbv7em-none-eabi)	9s
test (1.65.0)	1m 35s
build (stable, wasm32-unknown-unknown)	12s
test (stable)	14s
	5m 49s

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