inout: add `InOutBufReserved::split_reserved` method (#1133) #1764

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	19s
build (1.81.0, wasm32-unknown-unknown)	16s
build (stable, thumbv7em-none-eabi)	12s
build (stable, wasm32-unknown-unknown)	17s
test (1.81.0)	13s
test (stable)	12s
minimal-versions / minimal-versions	22s
	1m 51s

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