aead: factor apart `AeadInPlace`/`*Detached` (#1714) #2392

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	18s
build (1.85.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	17s
build (stable, wasm32-unknown-unknown)	12s
minimal-versions	41s
test (1.85.0)	25s
test (stable)	35s
	2m 47s

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