`bign-genk`: migrate to `KGenerator` api like `rfc6979` #228

Job	Run time
build (thumbv7em-none-eabi, stable)	19s
test (1.85.0)	21s
build (wasm32-unknown-unknown, stable)	20s
build (thumbv7em-none-eabi, 1.85.0)	19s
build (wasm32-unknown-unknown, 1.85.0)	18s
test (stable)	23s
	2m 0s

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