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rfc6979: integrate with `crypto-bigint`; `KGenerator` API #2752

Job	Run time
test (1.85.0)	35s
no_std (thumbv7em-none-eabi, 1.85.0)	36s
no_std (thumbv7em-none-eabi, stable)	34s
test (stable)	28s
no_std (wasm32-unknown-unknown, 1.85.0)	40s
no_std (wasm32-unknown-unknown, stable)	23s
	3m 16s

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