aead: factor apart `AeadInPlace`/`*Detached` (#1714) #2714

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	23s
build (1.85.0, wasm32-unknown-unknown)	18s
derive (1.85.0)	17s
build (stable, thumbv7em-none-eabi)	33s
derive (stable)	20s
build (stable, wasm32-unknown-unknown)	12s
minimal-versions	0s
test (1.85.0)	24s
test (stable)	15s
	2m 42s

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