aead: factor apart `AeadInPlace`/`*Detached` (#1714) #1961

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	15s
build (1.85.0, wasm32-unknown-unknown)	18s
test (1.85.0)	25s
build (stable, thumbv7em-none-eabi)	14s
test (stable)	21s
build (stable, wasm32-unknown-unknown)	18s
minimal-versions / minimal-versions	38s
	2m 29s

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