elliptic-curve: use `crypto_common::Generate` #3051

Job	Run time
build (1.85.0, wasm32-unknown-unknown)	21s
build (stable, wasm32-unknown-unknown)	11s
build (1.85.0, thumbv7em-none-eabi)	22s
test (stable)	14s
test (1.85.0)	20s
build (stable, thumbv7em-none-eabi)	24s
minimal-versions	0s
	1m 52s

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