x448: initial import #61

Job	Run time
test (1.85.0)	28s
test (stable)	30s
build (stable, thumbv7em-none-eabi)	33s
doc	36s
build (1.85.0, thumbv7em-none-eabi)	25s
build (1.85.0, wasm32-unknown-unknown)	28s
build (stable, wasm32-unknown-unknown)	22s
	3m 22s

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