elliptic-curve: add missing `Debug` impls (#1675) #2884

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	42s
build (1.73.0, wasm32-unknown-unknown)	42s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	49s
test (1.73.0)	32s
build (stable, wasm32-unknown-unknown)	36s
test (stable)	27s
test (nightly)	31s
	4m 19s

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