k12: new implementation with parallel processing support #1507

Job	Run time
minimal-versions / minimal-versions	27s
set-msrv / set-msrv	4s
test (1.85.0)	28s
build (stable, thumbv7em-none-eabi)	21s
test (stable)	27s
build (1.85.0, wasm32-unknown-unknown)	20s
build (stable, wasm32-unknown-unknown)	25s
build (1.85.0, thumbv7em-none-eabi)	24s
	2m 56s

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