inout: add `InOutBufReserved::split_reserved` method #659

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	13s
test (1.81.0)	22s
build (1.81.0, wasm32-unknown-unknown)	15s
test (stable)	20s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	9s
minimal-versions / minimal-versions	17s
	1m 49s

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