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`bign-genk`: migrate to `KGenerator` api like `rfc6979` #1721

Job	Run time
test (windows-latest, stable)	3m 43s
test (windows-latest, 1.85.0)	1m 35s
test (macos-latest, 1.85.0)	52s
build (thumbv7em-none-eabi, stable)	16s
test (ubuntu-latest, stable)	1m 20s
build (wasm32-unknown-unknown, stable)	20s
test (macos-latest, stable)	1m 4s
build (wasm32-unknown-unknown, 1.85.0)	21s
build (thumbv7em-none-eabi, 1.85.0)	20s
test (ubuntu-latest, 1.85.0)	51s
	10m 42s

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