aead: factor apart `AeadInPlace`/`*Detached` (#1714) #2281

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	29s
build (1.85.0, wasm32-unknown-unknown)	25s
build (stable, thumbv7em-none-eabi)	24s
minimal-versions	49s
build (stable, wasm32-unknown-unknown)	19s
test (1.85.0)	35s
test (stable)	28s
	3m 29s

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