ecdsa

rfc6979: integrate with `crypto-bigint`; `KGenerator` API #2742

Job	Run time
test (1.85.0)	13s
test (stable)	18s
no_std (thumbv7em-none-eabi, stable)	23s
no_std (thumbv7em-none-eabi, 1.85.0)	31s
no_std (wasm32-unknown-unknown, 1.85.0)	25s
no_std (wasm32-unknown-unknown, stable)	2s
	1m 52s

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