k12: add `Kt256`; rename `KangarooTwelve` to `Kt128` (#756) #1388

Job	Run time
set-msrv / set-msrv	5s
minimal-versions / minimal-versions	40s
test (stable)	35s
build (1.85.0, thumbv7em-none-eabi)	21s
build (stable, thumbv7em-none-eabi)	23s
test (1.85.0)	40s
build (stable, wasm32-unknown-unknown)	13s
build (1.85.0, wasm32-unknown-unknown)	24s
	3m 21s

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