refactor: simplify k generation following RFC 6979 #1427

Job	Run time
no-std build	37s
Unix tests (ubuntu-latest, stable)	12m 14s
WASM tests	4m 4s
Unix tests (ubuntu-latest, nightly)	12m 13s
Windows tests (stable, x86_64-pc-windows-gnu)	15m 3s
Unix tests (macos-13, stable)	12m 13s
Windows tests (stable, x86_64-pc-windows-msvc)	15m 6s
Unix tests (macos-13, nightly)	12m 13s
Windows tests (nightly, x86_64-pc-windows-gnu)	15m 5s
Unix tests (macos-14, stable)	12m 10s
Windows tests (nightly, x86_64-pc-windows-msvc)	15m 0s
Unix tests (macos-14, nightly)	12m 10s
	2h 18m 8s

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