rfc6979: add `generate_k_mut`; remove digest bounds #530

Job	Run time
build (thumbv7em-none-eabi, 1.71.0)	23s
build (thumbv7em-none-eabi, stable)	12s
test (ubuntu-latest, 1.71.0)	43s
build (wasm32-unknown-unknown, 1.71.0)	21s
build (wasm32-unknown-unknown, stable)	13s
test (ubuntu-latest, stable)	45s
test (macos-latest, 1.71.0)	1m 23s
test (macos-latest, stable)	1m 2s
test (windows-latest, 1.71.0)	1m 13s
test (windows-latest, stable)	1m 47s
	8m 2s

Provide feedback