elliptic-curve: add missing `Debug` impls #2205

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	50s
minimal-versions	39s
build (1.65.0, wasm32-unknown-unknown)	51s
build (stable, thumbv7em-none-eabi)	11s
build (stable, wasm32-unknown-unknown)	11s
test (1.65.0)	55s
test (stable)	13s
	3m 50s

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