rfc6979: integrate with `crypto-bigint`; `KGenerator` API #1190

Job	Run time
no_std (thumbv7em-none-eabi, stable)	49s
no_std (wasm32-unknown-unknown, 1.85.0)	51s
test (1.85.0)	3m 0s
no_std (thumbv7em-none-eabi, 1.85.0)	55s
test (stable)	2m 55s
no_std (wasm32-unknown-unknown, stable)	52s
	9m 22s

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