inout: add `InOutBufReserved::split_reserved` method (#1133) #1754

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	13s
build (1.81.0, wasm32-unknown-unknown)	14s
test (1.81.0)	14s
build (stable, thumbv7em-none-eabi)	8s
test (stable)	9s
build (stable, wasm32-unknown-unknown)	17s
minimal-versions / minimal-versions	16s
	1m 31s

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