A few optimizations on RFC6979 `generate_k_mut` #596

Job	Run time
build (thumbv7em-none-eabi, 1.72.0)	20s
test (ubuntu-latest, 1.72.0)	40s
build (thumbv7em-none-eabi, stable)	14s
build (wasm32-unknown-unknown, 1.72.0)	20s
test (ubuntu-latest, stable)	34s
build (wasm32-unknown-unknown, stable)	23s
test (macos-latest, 1.72.0)	1m 25s
test (macos-latest, stable)	50s
test (windows-latest, 1.72.0)	1m 16s
test (windows-latest, stable)	1m 59s
	8m 1s

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