aead: factor apart `AeadInPlace`/`*Detached` (#1714) #2770

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	20s
build (1.85.0, wasm32-unknown-unknown)	22s
test (1.85.0)	26s
test (stable)	20s
build (stable, thumbv7em-none-eabi)	15s
build (stable, wasm32-unknown-unknown)	20s
	2m 3s

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