inout

inout: add `InOutBufReserved::split_reserved` method #658

Job	Run time
test (1.81.0)	12s
test (stable)	18s
build (1.81.0, thumbv7em-none-eabi)	11s
build (1.81.0, wasm32-unknown-unknown)	13s
build (stable, thumbv7em-none-eabi)	7s
build (stable, wasm32-unknown-unknown)	9s
minimal-versions / minimal-versions	20s
	1m 30s

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