elliptic-curve: add missing `Debug` impls (#1675) #2580

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	16s
test (1.71.0)	26s
build (1.71.0, wasm32-unknown-unknown)	16s
test (stable)	17s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	12s
	1m 40s

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