A few optimizations on RFC6979 `generate_k_mut` #113

Job	Run time
test (1.72.0)	18s
build (thumbv7em-none-eabi, 1.72.0)	18s
test (stable)	23s
build (thumbv7em-none-eabi, stable)	14s
build (wasm32-unknown-unknown, 1.72.0)	19s
build (wasm32-unknown-unknown, stable)	13s
	1m 45s

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