elliptic-curve: add missing `Debug` impls #2103

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	57s
minimal-versions	21s
build (1.65.0, wasm32-unknown-unknown)	50s
test (1.65.0)	56s
build (stable, thumbv7em-none-eabi)	14s
test (stable)	13s
build (stable, wasm32-unknown-unknown)	25s
	3m 56s

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