k12: new implementation with parallel processing support #1506

Job	Run time
minimal-versions / minimal-versions	25s
set-msrv / set-msrv	3s
test (1.85.0)	37s
test (stable)	37s
build (1.85.0, wasm32-unknown-unknown)	37s
build (1.85.0, thumbv7em-none-eabi)	37s
build (stable, wasm32-unknown-unknown)	37s
build (stable, thumbv7em-none-eabi)	37s
	4m 10s

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