k12: new implementation with parallel processing support #880

Job	Run time
set-msrv / set-msrv	3s
minimal-versions / minimal-versions	30s
test (1.85.0)	28s
build (1.85.0, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	25s
test (stable)	31s
build (1.85.0, wasm32-unknown-unknown)	21s
build (stable, thumbv7em-none-eabi)	21s
	3m 2s

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