elliptic-curve: add missing `Debug` impls #2578

Job	Run time
test (1.71.0)	21s
build (1.71.0, thumbv7em-none-eabi)	22s
test (stable)	20s
build (1.71.0, wasm32-unknown-unknown)	18s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	18s
	1m 52s

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