elliptic-curve: add missing `Debug` impls #2881

Job	Run time
minimal-versions	0s
build (1.73.0, thumbv7em-none-eabi)	43s
build (1.73.0, wasm32-unknown-unknown)	37s
build (stable, thumbv7em-none-eabi)	43s
build (stable, wasm32-unknown-unknown)	34s
test (1.73.0)	29s
test (stable)	23s
test (nightly)	31s
	4m 0s

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